We're told over two months how many minutes of calls in that month, with how much credit was left. we can use that to build a system of equations.
33 min of calls left $24.72. Since we don't know what the starting credit was, we'll start there, then subtract 33 times the amount per minute, which gives us 24.72:
c - 33m = 24.72
We'll do that to the next set of data as well: 62 minutes left $20.08 in credit:
c - 62m = 20.08
Now we have two unknowns and two equations. I'll solve this by substituion. I'll set both equations to c in terms of m, then set the m terms equal to each other since they are both equal to the same "c". Then solve for m:
c - 33m = 24.72
c = 24.72 + 33m
c - 62m = 20.08
c = 20.08 + 62m
set them equal to each other and solve for m:
24.72 + 33m = 20.08 + 62m
4.64 = 29m
0.16 = m
So each minute costs 16 cents. Now we can solve for c:
c = 20.08 + 62m
c = 20.08 + 62(0.16)
c = 20.08 + 9.92
c = 30
Now we know they start with $30 in credit.
Now we can solve the final question: How much credit remains after 70 min of calls:
c - 70m = x
We know c and m. we can plug those in and solve for x:
30 - 70(0.16) = x
30 - 11.2 = x
x = 18.8
There will be $18.80 left in credit after 70 minutes.