Question

Charles owed $390 to his friend. On the first day Charles paid his friend $12. Each following week the amount Charles paid his friend increased by the same amount. After 10 payments, Charles had paid back the full amount. By how much did each payment increase?

Answer 1

The rate at which Charles amount increased will be given by:
Amount owe=$390
Payment=$10 
Number of payments=10
thus using the formula:
A=P(1+r)^n
plugging the values we get:
390=10(1+r)^10
solving for r we get
39=(1+r)^10
r=0.4425
thus the amount increased by 44.25%