Correct Answer - Option 2 : 21 L

**Given∶**

Initial ratio = 7 ∶ 5

9 liters of solution is taken out and the can is filled with HNO3

New ratio = 7 ∶ 9

**Formula Used∶**

Basics of Ratios and Linear equations.

**Calculation∶**

Let the containers initially contains 7x and 5x of HCL and HNO

3 respectively,

Quantity of HCL in solution left = [ 7x - 7/12 × 9 ] L = [ 7x - 21/4] L

Quantity of HNO3 in solution left = [ 5x - 5/12 × 9] L = [5x - 15/4] L

Resulting ratio,

⇒ [7x - 21/4] / [ 5x - 15/4] + 9 = 7/9

⇒ (28x - 21) / (20x + 21) = 7/9

⇒ 252 x - 189 = 140x + 147

⇒ 112x = 336

⇒ x = 336/112 = 3

Thus, Quantity of HCL = 7x = 7 × 3 = 21

**∴ The containers contained 21 liters of HCL.**