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

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