Explanation:

  Let, no. of boys and girls is b and g respectively.

∴ b + g = 52 -------------------(i)

Also, g - b = 2 -----------------(ii)

By (i) and (ii)

⇒ b = 25 ⇒ g = 27

⇒ Total weight of boys = 42 × 25 = 1050

⇒ Total weight of class = 52 × 40 = 2080

Weight of girls = Total weight of class – Total weight of boys

∴ Weight of girls = 2080 – 1050 = 1030

Average weight of girls = 1030/27

= 38.148 » 38