In a class, the average weight of 40 boys is 65 kg and that of 50 girls is 60 kg. After a few days, 40% of the girls and 50% of the boys le…

Solution
Initially, number of boys = 40 and number of girls = 50
Average weight of boys = 65 kg and average weight of girls = 60 kg
Now, 40% of the girls and 50% of the boys leave
=> Boys left = $\frac{100 - 50}{100} \times 40 = 20$
Girls left = $\frac{100 - 40}{100} \times 50 = 30$
Since, average weight of the boys and the girls remains constant throughout
$\therefore$ New average weight of the class
= $\frac{(20 \times 65) + (30 \times 60)}{20 + 30} = \frac{1300 + 1800}{50}$
= $\frac{3100}{50} = 62$ kg