Let \(d=\) total distance covered
\(V=\) average speed
\(r=\frac{d}{\text { Time }}=\frac{d}{1.5}\)
Time \(=1 hr 30 mins =1.5 hrs\)lso. when \(V\) was increased by \(20 km / hr\). we have \(V+20=\frac{d}{\text { Time }}\)
Time here becomes \(1 hr 15 mins =1.25 hrs\)fter \(15 mins\).
\(\Rightarrow r+20=\frac{d}{1.25}\).ividing (2) by (1) and note that the same distance is covered. we have
\begin{aligned}
\frac{1+20}{r} &=\frac{d}{1.25} \times \frac{1.5}{d} \\
\frac{r+20}{r} &=1.2(d \text { cancels }) \\
1+20 &=1.2 v \\2 v-r &=20 \\2 v &=20 \\
1 &=\frac{20}{0.2}=100 km / hr
\end{aligned}