The magnetic flux through a coil containing 50 loops changes from 30 Wb to –30 Wb in 0.42 seconds. What is the voltage induced in the coil?

Induced emf ${V}_{i . n . d} = - N \frac{\Delta \left(\Phi\right)}{\Delta t}$
$N$= number of turns;
$\Phi$= magnetic flux.
So:${V}_{i . n . d} = - 50 \frac{- 30 - 30}{0.42} = 7143 V$