Increase or decrease in percentage with price

If the price of a commodity increases by R%, then the reduction in consumption so as not to increase the expenditure will be,

$$\begin{aligned} \left[ \frac{R}{(100+R)}\times 100 \right]\% \end{aligned}$$

If the price of a commodity decreases by R%, then the increase in consumption so as not to decrease the expenditure will be

$$\begin{aligned} \left[ \frac{R}{(100-R)}\times 100 \right]\% \end{aligned}$$