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 leave. What would be the new average weight of the class (in kg)? Assume that the average weight of the boys and the girls remains constant throughout.

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