The population of a city is $20000$. It increased by $20\%$ during the first year and $30\%$ during the second year. The population after two years will be:

How much pure alcohol has to be added to $400 \text{ ml}$ of a solution containing $15 \%$ alcohol to change the concentration of alcohol in the mixture to $32 \%$?

If a number $x$ is $10 \%$ less than another number $y$ and $y$ is $10 \%$ more than $125,$ then $x$ is equal to

Two numbers are $30 \%$ and $40 \%$ more than the third number respectively. The first number is $x \%$ of the second. Then $x = ?$

In an examination,$37 \frac{1}{2} \%$ of the candidates were girls. If $75 \%$ of the boys and $62 \frac{1}{2} \%$ of the girls passed,and $342$ girls failed,find the number of boys who failed.

The radius of a sphere is increased by $10 \%$. The surface area increases by