Number of boys = 4/5 × 150 = 120 ⇒ Number of girls = 1/5 × 150 = 30 ⇒ Mean proportional between the number of boys and girls = √( Number of boys × Number of girls) ⇒ Mean proportional between the number of boys and girls = √(120 × 30 = 60 ∴ Mean proportional between the number of boys and girls is 60.