A solenoid 1.5m long and 0.4 cm diameter possesses 10 turns per cm length. A current of 5A flows through it. The magnetic field at the axis inside the solenoid is $n\pi \times {10}^{-3}T$ where $n=\_\_\_\_$  

Here length of solenoid is far greater than the diameter hence we consider it is an infinite length of solenoid
Magnetic induction inside the solenoid
$\begin{array}{l}B={\mu }_I where n=10/cm\\ =4\pi \times {10}^{-7}\times {10}^3\times 5=10/{10}^{-2}m\\ =20\pi \times {10}^{-4}={10}^3/m\\ B=2\pi \times {10}^{-3}T\end{array}$