Correct Answer - Option 3 : 80 cm

**Given :**

The area of a trapezium is 2400 sq.cm and the sum of the parallel sides is three times the height and one of the parallel sides is 40cm.

**Concept used :**

Area of a trapezium = (1/2) × (sum of parallel sides) × Height

**Calculations :**

Let the length of other side be 'l' cm

40 + l = 3 × Height

Height = (40 + l)/3

Area of trapezium = (1/2) × (40 + l) × (40 + l)/(3)

(40 + l)^{2} = 2400 × 2 × 3

⇒ (40 + l)^{2} = 14400

⇒ 40 + l = √14400

⇒ l + 40 = 120

⇒ l = 80

**∴ Option 3 will be the correct answer.**