$A$ student multiplied a number by $\frac{3}{5}$ instead of $\frac{5}{3}$. What is the percentage error in the calculation (in $\%$)?

The population of a town is decreased by $20 \%$ and $25 \%$ in two successive years. What per cent population is decreased after two years?

The sum of two numbers is $\frac{23}{20}$ of the first number. The second number is what percent of the first?

The price of cooking oil has increased by $25 \%$. The percentage of reduction that a family should effect in the use of cooking oil,so as not to increase the expenditure on this account,is (in $\%$)

If the height of a triangle is decreased by $40 \%$ and its base is increased by $30 \%$,what will be the effect on the area?

Water tax is increased by $20 \%$ but its consumption is decreased by $20 \% .$ Then the increase or decrease in the expenditure of the money is