Two ways to get Robux for Roblox. Your mission: figure out which option saves money — and when the winner changes.
The amount of Robux I buy is the because I get to choose it.
The total cost is the because it changes based on how many Robux I pick.
In y = kx, the letter k tells me the — how much cost goes up per Robux.
In this activity, x = because that is what I choose.
In this activity, y = because it depends on how many Robux I buy.
As x (Robux) goes up, Plan B's cost goes but Plan A's cost .
| Robux — x | Plan A Cost ($) | Plan B Cost ($) |
|---|---|---|
| 200 | ||
| 400 | ||
| 600 | ||
| 800 | ||
| 1000 |
Both plans cost the same at around Robux, where both equal $.
At 200 Robux, Plan is cheaper by $.
k = $0.0125 means for every 1 Robux I buy, the total cost goes up by $.
Plan A's cost is always $10, so Plan A is — the cost doesn't change with x.
Use Plan B's equation to find the cost for 600 Robux:
y = 0.0125 × 600 = $. This matches my table row for 600 Robux. ✓
What shape does Plan A's line make, and why?
Plan A makes a line because its cost
What shape does Plan B's line make, and what does that tell us?
Plan B makes a line that goes because each extra Robux adds $
Do the lines cross? If yes, at approximately what Robux amount?
Yes / No — the lines look like they cross near Robux
What does the crossing point mean in this situation?
At that crossing point, both plans cost the same ($). Before it, is cheaper. After it, is cheaper.
If I want fewer than Robux, I should use Plan because it costs less.
If I want more than Robux, I should use Plan because it costs less.
My table shows: at 800 Robux, Plan A = $ and Plan B = $, so Plan is cheaper by $.
My graph shows: Plan B's line goes from left to right, which means more Robux = cost for Plan B.
In this activity, x = ___ because ___ . y = ___ because it depends on x.
Plan B's equation is y = ___ x. The k value of ___ means that for every 1 Robux, the cost goes up by $___ .
For a friend who buys 600 Robux per month, I recommend Plan ___ because my table shows ___ and my graph shows ___ .
Two ways to get Robux for Roblox. Your mission: figure out which option saves money — and when the winner changes.
What is the independent variable? Write a complete explanation.
The independent variable is because I get to choose it. It doesn't depend on anything else —
What is the dependent variable? Explain why it depends on x.
The dependent variable is because it changes based on how many Robux I buy.
If x goes up, y goes for Plan B because each Robux costs $.
| Robux — x | Plan A ($) | Plan B ($) | Cheaper Plan |
|---|---|---|---|
| 200 | |||
| 400 | |||
| 600 | |||
| 800 | |||
| 1000 | |||
Describe the pattern as Robux increases. What happens to each plan's cost and why?
As Robux increases, Plan A's cost because .
Plan B's cost because .
They behave differently because Plan A is while Plan B is .
What is Plan B's k value and what does it mean in context?
Plan B's k = $. This means for every 1 Robux I buy, the total cost goes up by $.
So k is the — it tells me how quickly cost grows as Robux increases.
Is Plan B a proportional relationship? Use both the equation and table to explain.
Plan B proportional because its equation is y = 0.0125x, which is in form.
In my table, when x doubles from 200 to 400, y also from $ to $.
Use Plan B's equation to find the cost for 750 Robux:
y = 0.0125 × 750 = $. So 750 Robux costs $ with Plan B.
Give the approximate coordinate where the two lines cross:
The lines cross at approximately ( Robux , $ )
What does that crossing point mean for someone choosing between Plan A and Plan B?
At that point, both options cost $. If you need fewer than Robux, use because it's cheaper.
If you need more than Robux, switch to because
Describe Plan A's line shape and explain it using the equation:
Plan A's line is because its equation is y = , which means the cost
never / always changes as x increases.
What does the steepness of Plan B's line tell you about its k value?
The steeper the line, the the k value. {plan_b}'s k = , so every unit of x adds $ to the cost.
My table shows that the Cheaper Plan column switches from Plan to Plan at around Robux.
At 800 Robux, Plan A = $ and Plan B = $, a difference of $.
Plan B's equation y = 0.0125x shows k = $0.0125 per Robux, meaning every Robux adds $ to the cost.
Plan A is always $10, so Plan B is cheaper when x is than Robux.
On my graph, the lines cross at about ( Robux, $ ).
To the left of that point, Plan is cheaper. To the right, Plan is cheaper.
For a player who buys about Robux per month, I recommend Plan because
Plan B's k = $0.0125 per Robux. Plan A's cost never changes. This tells me that Plan B grows ___ expensive faster because ___ .
My graph's crossing point is at ( ___ Robux, $___ ). To the left, use Plan ___ . To the right, use Plan ___ because ___ .
If someone buys 500 Robux/month, the cost with Plan B = 0.0125 × 500 = $___ . Compared to Plan A's $___ , Plan ___ saves $___ .
Plan C costs $5/month + $0.008 per Robux. Its equation is y = 0.008x + 5. What is Plan C's k value? Use the equation to find the cost for 500 Robux, then add Plan C to your graph.
Plan C's k = $ per Robux. At 500 Robux: y = 0.008 × 500 + 5 = $ + $5 = $.
On my graph, Plan C starts at y = $ and rises than Plan B.
Three ways to get Robux. Compare all three — find out which saves the most money and when the winner changes.
Before calculating: predict which plan is cheapest at 200 Robux and at 800 Robux. Use vocabulary terms.
At 200 Robux I predict Plan is cheapest because its rate of change (k) seems .
At 800 Robux I predict Plan wins because .
Do you think one plan is always cheapest, or does the winner change?
I think the winner because Plan A has a cost while Plan B has a rate of change.
| Robux — x | Plan A ($) | Plan B ($) | Plan C ($) | Winner |
|---|---|---|---|---|
| 200 | ||||
| 400 | ||||
| 625 | ||||
| 800 | ||||
| 1000 |
Did your results match your prediction? What surprised you?
My prediction was because .
I was surprised that at Robux, Plan was cheaper than I expected because .
The Winner column changes times, which means there are different zones.
List and compare the k values for Plans B and C. What does a smaller k value mean?
Plan B: k = $ per Robux. Plan C: k = $ per Robux.
Plan C has a k, which means its cost grows as Robux increases.
A smaller k = lower rate of change, so for very large Robux amounts, Plan will eventually have the smaller fee.
Which plans are proportional? Which are not? Explain using the equation form.
Plan B is proportional because its equation is y = 0.0125x — it's in form.
Plan C is proportional because of the + 5 — even at x = 0 Robux, you'd still pay $.
Plan A is proportional because y always equals $10 regardless of x.
Use Plan C's equation to find the cost for 500 Robux:
y = 0.008 × 500 + 5 = $ + $5 = $. So 500 Robux costs $ with Plan C.
Plan C charges $13. Use the equation y = 0.008x + 5 to find x. Steps are set up for you:
Start: 13 = 0.008x + 5. Subtract 5 from both sides: = 0.008x.
Divide both sides by 0.008: x = Robux. So $13 buys Robux with Plan C.
Where do Plan A and Plan B cross? What does that mean?
Plan A and Plan B cross at about ( Robux , $ ).
At that point both cost exactly $. Before it is cheaper; after it is cheaper.
Where do Plan A and Plan C cross? What does that mean?
Plan A and Plan C cross at about ( Robux , $ ).
This means at that point both cost exactly $. Before it is cheaper; after it is cheaper.
Describe the three zones your graph creates — which plan is cheapest in each?
Zone 1 (0 to Robux): is cheapest because its line is the lowest here.
Zone 2 ( to Robux): wins because
Zone 3 (above Robux): is cheapest because
Why is the graph more useful than just the table for finding these zones?
The graph shows all three lines at once, so I can see the crossing points at a glance. With only the table, I would have to
The Winner column changes times, creating zones. Plan wins at small Robux amounts (under ).
Plan B: k = $0.0125 (fastest growing). Plan C: k = $0.008 (slower). Plan A: flat $10.
Because Plan B's k is than Plan C's k, Plan B becomes more expensive .
My graph has crossing points, creating zones.
Zone 1 (0 to Robux): Plan is cheapest.
Zone 2 ( to Robux): Plan wins.
Zone 3 (above Robux): Plan is cheapest because .
Buy fewer than Robux → use Plan .
Buy between and Robux → use Plan .
Buy more than Robux → use Plan because .
Plan B's k = $0.0125 and Plan C's k = $0.008. This means Plan ___ grows more expensive faster because ___ . For very large Robux amounts, Plan ___ will become the best deal because ___ .
At 625 Robux: Plan A = $___ , Plan B = 0.0125 × 625 = $___ , Plan C = 0.008 × 625 + 5 = $___ . The cheapest plan at 625 Robux is Plan ___ .
My graph has ___ zones. Zone 1 ( ___ to ___ Robux ): Plan ___ cheapest. Zone 2: Plan ___ cheapest. Zone 3: Plan ___ cheapest.