In series $L-C-R$ circuit, the source voltage
Here, $V =\sqrt{V_{R}^{2}+\left(V_{L}-V_{C}\right)^{2}} $
$V_{R} =60 V \Rightarrow V_{L}=40\, V $
$V_{C} =120\, V $
$V =\sqrt{(60)^{2}+(40-120)^{2}} $
$=\sqrt{(60)^{2}+(-80)^{2}}=\sqrt{3600+6400} $
$=\sqrt{10000}=100 \,V$