In a class, the average weight of 80 boys is 64 kg and that of 75 girls is 70 kg. After a few days, 60% of the girls and 30% of the boys le…

Solution
Initially, number of boys = 80 and number of girls = 75
Average weight of boys = 64 kg and average weight of girls = 70 kg
Now, 60% of the girls and 30% of the boys leave
=> Boys left = $\frac{100 - 30}{100} \times 80 = 56$
Girls left = $\frac{100 - 60}{100} \times 75 = 30$
Since, average weight of the boys and the girls remains constant throughout
$\therefore$ New average weight of the class
= $\frac{(56 \times 64) + (30 \times 70)}{56 + 30} = \frac{3584 + 2100}{86}$
= $\frac{5684}{86} = 66.09$ kg