Question

Let P(n) be the statement "Any collection of n coins can be obtained using a combination of 3 and 5 cent coins." Prove P(n) for n greater than or equal to 14.
n=14 P(n) = 3+3+3+5
n=15 P(n) = 5+5+5
n=16 P(n) = 3+3+5+5
How do I prove this in a general way?