Answer: 
Step-by-step explanation:
See attached image, it's direct formula substitution.
PEMDAS so (9+15)= 24 , 24/3 = 8 , 8+2 = 10
F(x) = cos x
f'(x) = -sin x
f''(x) = -cos x
f'''(x) = sin x
f''''(x) = cos x
90/4 = 22 Remainder 2
Therefore, 90th derivative of f(x) = cos x is -cos x.
It's not compounded interest, you just get $3.20 every year.
$100 * 0.032 = $3.20
$3.20 * 3 years = $9.60