Correct Answer - Option 1 : 42

Given:

Average marks of 3 girls = 50

The ratio of the marks Girl1 ∶ Girl2 = 5 ∶ 7

Marks of Girl3 = Marks of Girl1 – 3

Formula:

Average = Sum of all observations/Total number of observations

Calculation:

Total marks of 3 girls = 3 × 50 = 150

Let the marks of Girl1 be 5x and Girl2 be 7x

Girl3 = 5x – 4

Total marks = 5x + 7x + 5x – 3

⇒ 150 = 17x – 3

⇒ 17x = 153

⇒ x = 153/17 = 9

Marks of Girl_{1} = 5x = 45

Girl_{3} = Marks of Girl1 – 3 = 45 – 3 = 42

**∴ Marks of Girl**_{3} is 42