A coil is placed perpendicular to a magnetic field of 5000T . When...

Average induced EMF is given by, $ε = N\left(\dfrac{∆ ϕ}{t}\right)$

Also, change in flux $∆ ϕ = ( ∆ B ) A$

Given: ${B}_{i} = 5000T$,

${B}_{f} = 3000T$

$d = 0 . 02m ⇒ r = \dfrac{d}{2} = 0 . 01m$

Hence, 

$∆ ϕ = ( ∆ B ) A$

$= \left(2000\right)\pi{\left(0 . 01\right)}^{2} = 0 . 2\pi$

EMF, $ε = N\left(\dfrac{∆ ϕ}{t}\right) ⇒ 22 = N\left(\dfrac{0 . 2\pi}{2}\right)$

$⇒ N = 70$