Answer
$2.2\times10^3$ turns
Work Step by Step
Use equation 20–8 for the magnetic field inside a solenoid.
$$B=\frac{\mu_oNI}{\mathcal{l}}$$
$$N=\frac{B\mathcal{l}}{\mu_oI}$$
$$=\frac{(0.030T)(0.42m)}{(4\pi \times10^{-7} T\cdot m/A)(4.5A)} $$
$$=2.2\times10^3 turns$$
