Question

In a class of 100 students there are 70 boys whose average marks in a subject are 75. If the average marks of the complete class are 72, then the average marks of the girls :

• A. $73$
• B. $65$
• C. $68$
• D. $74$

Answer 1

The correct option is B $65$

Average of marks of $70$ boys =$75$

Sum of marks of $70$ boys = $75\times 70$ = $5250$

Average of marks of whole class = $72$

Sum of marks of whole class = $100\times 72$ = 7200

Sum of marks of $30$ girls =$7200-5250=1950$

Average of marks of girls = $\frac{1950}{30}$ =$65$