You could set up a proportion, using symbol x to represent the unknown number of seconds they ask for. There's also another way to think about it, using a rate to see a pattern.
If you're in algebra class, then maybe you're supposed to write and solve an equation. Here's a worked example that's similar, using a proportion. If I eat 4 apples per 30 days, how many days do I need to eat a dozen apples?
Let x = the number of days it takes to eat 12 apples
4 apples per 30 days is a rate
12 apples per x days is the same rate, yes? (I haven't changed how fast I'm eating apples; I'm just writing the rate in an equivalent way.) Because the rates are equal, I can write a proportion.
\(\displaystyle \dfrac{4}{30} = \dfrac{12}{x}\)
Solve for x.
Using the rule I learned in prealgebra (for solving proportions when you know three of the four numbers), I multiply on the diagonal and divide by the number not used.
x = (30 × 12) / 4
4 goes into 12 three times, so x = 30 × 3
I need 90 days, to eat a dozen apples (at the rate of 4 apples every 30 days).
You could set up and solve a proportion for your exercise similarly.
There's also an alternate rule for solving proportions.
a/b = c/d means a∙d = b∙c
4/30 = 12/x means 4x = 30(12)
Divide each side by 4. This way is basically the same as what I did above using the prealgebra rule.
With proportions, you can also switch to using reciprocals.
30/4 = x/12
Reduce 30/4 and then multiply each side by 12.
I get x = 6(15)
If you haven't learned about proportions or you're not yet solving equations in beginning algebra, then here's another way: 2 tokens per 30 seconds
2/30 reduces to 1/15
So, the rate at which Jenny gets tokens may also be expressed as: 1 token per 15 seconds
We need the time for 5,000 tokens. Writing out simpler forms of what you're asked to find, is often a good way to consider word problems. Let's write some times for smaller token numbers.
1 token
s : 15sec
2 tokens : 15sec + 15sec
3 tokens : 15sec + 15sec + 15sec
4 tokens : 15sec + 15sec + 15sec + 15sec
The pattern is easy to see. We need the same number of 15-second intervals as tokens.
If we want 4 tokens, we need 4×15 seconds.
If we want 100 tokens, we need 100×15 seconds.
What if we want 5000 tokens?