Two identical tanks R and S contained some water. Tank R was ¹₈ filled and Tank S was ¹₉ filled. The ratio of the total mass of Tank R and the water in it to the total mass of Tank S and the water in it was 11 : 10. The total mass of the two tanks and the water they contained was 5670 g. What is the mass of an empty tank?
Mass of Tank R and the water inside = 11 u
Mass of Tank S and the water inside = 10 u

Total mass of the two tanks and the water inside them
= 11 u + 10 u
= 21 u

21 u = 5670

1 u = 5670 ÷ 21 = 270

Mass of Tank R and the water in it more than mass of Tank S and the water in it 
= 11 u - 10 u
= 1 u

Fraction of the water that Tank R had more than Tank S 
= 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
= 10 u - 8 u
= 2 u
= 2 x 270
= 540 g

Answer(s): 540 g