Let An be the number of ways that nickels and dimes can be inserted (one at a time) into a vending machine to purchase and item that costs 5n cents.

a) Find a1, a2, a3, a4

b) Find a recurrence relation for An

I think a1=5 a2=10 a3=15 and a4=20

but i cant find an answer for b.

Can someone please help?

Thank you