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 13 : 12. The total mass of the two tanks and the water they contained was 6000 g. What is the mass of an empty tank?
Mass of Tank R and the water inside = 13 u
Mass of Tank S and the water inside = 12 u

Total mass of the two tanks and the water inside them
= 13 u + 12 u
= 25 u

25 u = 6000

1 u = 6000 ÷ 25 = 240

Mass of Tank R and the water in it more than mass of Tank S and the water in it 
= 13 u - 12 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
= 12 u - 8 u
= 4 u
= 4 x 240
= 960 g

Answer(s): 960 g