# Number of turns for a solenoid

## Homework Statement

The magnetic field in the middle of a solenoid of length L= 40cm and area of the cross-section A=12 cm^2 is B = 3 mT. The current through the solenoid is I = 4 A. What is the number of turns in this solenoid?

BL=μ0*n*I
=(4πx10^-7)*n*I

## The Attempt at a Solution

I have attempted this problem many times trying to use the formula above or similar variations of it with no success. I need to use the area of the cross-section in a specific formula to calculate the number of turns. Is there a formula that I'm missing as it is not in my notes, if not how would I proceed?