Your friend asks you to help him study and will pay you 5 dimes the first time you help him. You agree to help if he multiplies your payment by 5 for each study session. After 2 study sessions, you will receive 25 dimes, and after 3 study sessions, you will receive 125 dimes. Complete and solve the equation that finds the number of dimes he will pay you after the 7th study session.

a_n = 5^n