If $200 \%$ of a number is $90,$ then what is the $80 \%$ of that number?

In measuring the sides of a rectangle,one side is taken $10 \%$ in excess and the other $20 \%$ in deficit. Find the error per cent in area calculated from the measurement.

The price of petrol went up by $20 \%$. In order to keep expenses the same,Ram must reduce travel by: (in $\%$)

Rita invested $25 \%$ more than Sunil. Sunil invested $30 \%$ less than Abhinav,who invested $Rs$ $6,000$. What is the ratio of the amount that Rita invested to the total amount invested by all of them together?

The tax on a commodity is diminished by $10 \%$ and its consumption increases by $10 \%$. Find the effect on revenue.

An agent gets a commission of $2.5 \%$ on the sales of cloth. If on a certain day,he gets Rs. $12.50$ as commission,the cloth sold through him on that day is worth?