A $90cm$ long solenoid has six layers of windings of $450$ turns each. If the diameter of solenoid is $2.2cm$ and current carried is $6A$, then the magnitude of magnetic field inside the solenoid, near its center is

For six layers of windings the total number of turns$=6\times 450=2700$

Now number of turns per unit length

$n=\frac{N}{I}=\frac{2700}{90\times {10}^{-2}}=3000$

Then the field inside the inside the solenoid near the center

$B={\mu }_{0}nI=4\mathrm{\pi }\times {10}^{-7}\times 3000\times 6=72\mathrm{\pi }\times {10}^{-4}\mathrm{T}=72\mathrm{\pi G}$