if $8000 is invested in a 2.5% account for 10 years compounded quartly how long would it take for the account to double?

Question

I don't know how to do this question

Answer 1

P = C (1 + r/n)^nt

C=$8000

r=.025

t=10 years 

n=4 (quarterly )

P=$8000(1+.025/4)^40

P=$10265