Question:
In the new budget, the price of petrol rose by 10%. By how much percent must one reduce the consumption so that the expenditure does not increase?
Solution:
We have to reduce the consumption such that the expenditure does not increase.
For this, we use the following formula:
$\left(\frac{r}{r+100}\right) \times 100$, where $\mathrm{r}=$ percentage rise in the price of the commodity
$\therefore$ Percentage reduction in the consumption $=\left(\frac{10}{10+100}\right) \times 100=\left(\frac{10}{110}\right) \times 100 \quad(\because r=10 \%)$
$=9 \frac{1}{11}$