In an examination,it is required to multiply a number by $7$,but by mistake,a student divides it by $7$. What is the error percentage?

The average marks obtained in a class of $50$ students is $70 \%$. The average of the first $25$ students is $60 \%$ and that of the next $24$ students is $80 \%$. What is the percentage of marks obtained by the last student?

In a college election between two candidates,one candidate got $55 \%$ of the total valid votes. $15 \%$ of the votes were invalid. If the total votes were $15,200$,what is the number of valid votes the other candidate got?

The income of a company increases $20 \%$ per annum. If its income is $Rs$ $42664000$ in the year $1999$,what was its income in the year $1997$?

Ram borrowed $Rs. 725$ from Shyam at the beginning of a year at a certain rate of interest. After $8$ months,he again borrowed $Rs. 362.50$ at a rate of interest double that of the former sum. At the end of the year,the total interest on both loans is $Rs. 43.50$. Find the first rate of interest per annum.

From a container having pure milk,$20 \%$ is replaced by water and the process is repeated thrice. At the end of the third operation,the milk is: (in $\%$ pure)