Tim started with $65.
First, we need to find the slope (or increase). From 13 to 18, the amount increased by $31.25. Divide that by 5 and we have an increase of 6.25 per ticket.
Now, just write and solve the following equation for our answer.
y = 6.25x + b
108.75 = 6.25(7) + b
108.75 = 43.75 + b
65 = b