If the price of the petrol is increased by 30%, by how much percent a car owner must reduce his…

To keep spending the same after a price rise of $r\%$, consumption must fall by $\frac{r}{100+r}\times100\%$. Here that is $\frac{30}{130}\times100=\frac{300}{13}=23\tfrac1{13}\%$. The cut is less than 30% because it is reckoned against the new, higher price; check: $1.3\times\left(1-\frac{3}{13}\right)=1.3\times\frac{10}{13}=1$.