Suppose you wish to make a solenoid whose self-inductance is 1.2 mH. The inductor is to have a cross-sectional area of 1.2 10-3 m2 and a length of 0.048 m. How many turns of wire are needed?

Answer 1

L= mu*N^2*A/l

you have everything, assume air core for mu, and solve for N