“A rectangular tank of base 45 cm by 60 cm was filled with water to a height of 36 cm“.

That was stated in the question.

“…the water level rose by 1/12 of its original height.”

The “its” refers to the water level (ie original height 36 cm), not the tank’s actual height.

¹/₁₂ of 36 cm = 3 cm

Volume of 6 mugs of water -> 45 × 60 × 3 = 8100 cm³

Volume of 1 mug of water -> 8100 ÷ 6 = 1350 cm³

Volume of 4 mugs of water -> 1350 × 4 = 5400 cm³

The total height is not 36cm then y taken 1/12th of 36

“A rectangular tank of base 45 cm by 60 cm was filled with water to a height of 36 cm“.

That was stated in the question.

“…the water level rose by 1/12 of its original height.”

The “its” refers to the water level (ie original height 36 cm), not the tank’s actual height.

6 mugs –> 1/12 of height 36cm = 3cm

1 mug –> 0.5 cm

4 mugs –> 2 cm –> 45x60x2 = 5400 cm³#