Two identical tanks P and Q contained some water. Tank P was ¹₇ filled and Tank Q was ¹₈ filled. The ratio of the total mass of Tank P and the water in it to the total mass of Tank Q and the water in it was 12 : 11. The total mass of the two tanks and the water they contained was 2760 g. What is the mass of an empty tank?
Mass of Tank P and the water inside = 12 u
Mass of Tank Q and the water inside = 11 u

Total mass of the two tanks and the water inside them
= 12 u + 11 u
= 23 u

23 u = 2760

1 u = 2760 ÷ 23 = 120

Mass of Tank P and the water in it more than mass of Tank Q and the water in it 
= 12 u - 11 u
= 1 u

Fraction of the water that Tank P had more than Tank Q 
= ¹₇ - ¹₈
= ⁸₅₆ - ⁷₅₆
= ¹₅₆ 

¹₅₆ of the water capacity → 1 u
⁷₅₆ of the water capacity → 7 x 1 u = 7 u

Mass of an empty tank
= 11 u - 7 u
= 4 u
= 4 x 120
= 480 g

Answer(s): 480 g