The field inside a solenoid

Three long solenoids have $800$, $1600$ and $2400$ turns in each metre. You will make the program find the field inside each at $2.5\ \text{A}$, the current that gives $6\ \text{mT}$ in the middle one, and the turns in each metre of a coil of $450$ turns that is $0.18\ \text{m}$ long. Then it draws the field against the current.

The idea. $B = \mu_0 n I$, with $\mu_0 = 4\pi \times 10^{-7}\ \text{T m/A}$ and $n$ the turns in each metre.

Your tasks.

1. Inside the loop, work out B, the field in millitesla at $2.5\ \text{A}$ for the solenoid with n turns in each metre.
2. Find I6, the current in amperes that gives $6\ \text{mT}$ when $n = 1600$.
3. Find n_coil, the turns in each metre of $450$ turns in $0.18\ \text{m}$.