Thank you for reporting, we will resolve it shortly
Q.
Magnetic flux passing through a coil is initially $4 × 10^{- 4}\, Wb$. It reduces to $10\%$ of its original value in ‘$t$’ second. If the e.m.f. induced is $0.72 \,mV$ then ‘$t$’ in second is
Magnetic flux gets reduced to $10 \%$ of its original value in time $t$.
Change in magnetic flux $|\Delta \phi|=0.9 \phi_{\text {initial }}$
$\therefore |\Delta \phi|=0.9 \times 4 \times 10^{-4}=0.36 \times 10^{-3} Wb$
Emf induced in the coil $E =0.72 \times 10^{-3} V$
Using $E =\frac{|\Delta \phi|}{ t }$
$\therefore 0.72 \times 10^{-3}=\frac{0.36 \times 10^{-3}}{ t }$
$\Rightarrow t =0.5 \,s$