A coil of mean area 500 cm² and having 1000 turns is held perpendicular to a uniform field of 0.4 gauss. the coil is turned through 180^o in 1/10 second. The average induced emf (in V) is

When the plane of coil is perpendicular to field, the angle between area $\overrightarrow{A}$ and field $\overrightarrow{B}$ is 0^o.

The flux linked with coil ${\varphi }_1=NBA\cos 0^0=NBA$. 
$when the coil rotates {180}^{0 }flux linked {\phi }_2=BAN\cos {180}^0=-BAN$

$\therefore \text{ Change in flux }\varphi ={\varphi }_2-{\varphi }_1=-2NBA$

$\text{ The magnitude of the induced emf is }\epsilon =-\frac{d\varphi }{dt}=\frac{2NBA}{dt}=\frac{2\times 1000\times 0.4\times {10}^{-4}\times 500\times {10}^{-4}}{{\displaystyle \frac{1}{10}}}=0.04V$