Correct Answer - Option 3 : 900

**Given:**

The ratio of CP of A and SP of B is 21 : 20.

The ratio of CP of B and SP of A is 14 : 15.

Total SP of both items is Rs. 100 more than the total CP.

On selling B. Loss of Rs. 800 was incurred.

**Formula used:**

Profit or Gain = Selling price – Cost Price

Loss = Cost Price – Selling Price

Profit percentage = (Profit /Cost Price) x 100

Loss percentage = (Loss / Cost price) x 100

**Calculation: **

Let the CP of A = 21x

SP of B = 20x

Let CP of B = 14y

SP of A = 15y

15y + 20x = 100 + 21x + 14y

⇒ Y = 100 + x ----(i)

14y – 20x = 800

⇒ 14(100 + x) – 20x = 800 [From (i)]

⇒ 1400 + 14x - 20x = 800

⇒ - 6x = - 600

⇒ x = 100

From equation (i),

y = 100 + x

⇒ y = 100 + 100

⇒ y = 200

CP of A = Rs. (21 × 100) = Rs. 2100

SP of A = Rs. (15 × 200) = Rs. 3000

Gain on selling A = Rs. (3000 - 2100) = Rs. 900

**∴ gain on selling A is Rs. 900**