Two identical tanks Y and Z contained some water. Tank Y was ¹₈ filled and Tank Z was ¹₉ filled. The ratio of the total mass of Tank Y and the water in it to the total mass of Tank Z and the water in it was 12 : 11. The total mass of the two tanks and the water they contained was 6440 g. What is the mass of an empty tank?
Mass of Tank Y and the water inside = 12 u
Mass of Tank Z 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 = 6440

1 u = 6440 ÷ 23 = 280

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

Fraction of the water that Tank Y had more than Tank Z 
= ¹₈ - ¹₉
= ⁹₇₂ - ⁸₇₂
= ¹₇₂ 

¹₇₂ of the water capacity → 1 u
⁸₇₂ of the water capacity → 8 x 1 u = 8 u

Mass of an empty tank
= 11 u - 8 u
= 3 u
= 3 x 280
= 840 g

Answer(s): 840 g