Answer
$4.7\times10^{20}$ electrons.
Work Step by Step
$I=\frac{q}{t}\implies q=It=50\,A\times1.5\,s=75\,C$
Number of electrons=$\frac{q}{e}=\frac{75\,C}{1.6\times10^{-19}\,C/electron}$$=4.7\times10^{20}\,electrons$
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