There are 50 students in a class of which 40 are boys and the rest girls. The average weight of the students in the class is 44 kg and the average weight of the girls is 40 kg. Find the average weight of boys.

No. of girls = Total - No. of boys = 50 - 40 = 10.

Average weight of class = $\dfrac{\text{Total weight}}{\text{No. of students}}$

∴ Total weight = Average weight × No. of students
⇒ Total weight = 44 × 50 = 2200

Average weight of girls = $\dfrac{\text{Weight of girls}}{\text{No. of girls}}$

∴ Weight of girls = Average weight of girls × No. of girls
⇒ Weight of girls = 40 × 10 = 400.

Total weight of boys = Total weight - Weight of girls = 2200 - 400 = 1800.

Average weight of boys = $\dfrac{\text{Weight of boys}}{\text{No. of boys}} = \dfrac{1800}{40}$ = 45.

Hence, the average weight of boys is 45 kg.