Given: Total boys = 20 Total girls = 30 Average marks of boys = 60 Average marks of girls = 50 Concept: Average = Total sum of marks/ Total persons Calculation: Sum of marks of boys = 20 × 60 ⇒ 1,200 Sum of marks of girls = 30 × 50 ⇒ 1,500 Average marks of whole class = (1,200 + 1,500)/ (20 + 30) = 2,700/50 = 54