I need help ! (sinusoidal models word problems)

I am not a native, my english is poor sorry.

this is problem in sinusoidal models word problems practice.

C(t) = -15cos(2pi/15*t)+65

t = 15/2pi(2.3005 + 2pi*n)

t is about 5.5 + 15n

Why 15/2pi * 2.3005 is 5.5 here?

I did this on calculater the answer is 54.20425...

It's not clearon text so I add screenshot.