Answer
N=20
Work Step by Step
(a) We know that
$A=\frac{r^3}{(1.2\times 10^{-15})^3}$
We plug in the known values to obtain:
$A=(\frac{4.0\times 10^{-15}}{1.2\times 10^{-15}})^3$
$\implies A=37$
Now we can find the number of neutrons as follows:
$N=A-Z$
$\implies N=37-17=20$