$x \%$ of $y + ? \%$ of $x = x \%$ of $(x + y)$

$? \times 15 = 37.5 \%$ of $220$

Six-elevenths of a number is equal to $22 \%$ of a second number. The second number is equal to one-fourth of a third number. The value of the third number is $2400$. What is $45 \%$ of the first number?

In a certain school,$20 \%$ of students are below $8$ years of age. The number of students above $8$ years of age is $2/3$ of the number of students of $8$ years of age,which is $48$. What is the total number of students in the school?

In an examination,$65 \%$ of students passed in Civics and $60 \%$ in History,while $40 \%$ passed in both of these subjects. If $90$ students failed in both History and Civics,then what is the total number of students?

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.