Two identical tanks S and T contained some water. Tank S was ¹₉ filled and Tank T was ¹₁₀ filled. The ratio of the total mass of Tank S and the water in it to the total mass of Tank T and the water in it was 13 : 12. The total mass of the two tanks and the water they contained was 6500 g. What is the mass of an empty tank?
Mass of Tank S and the water inside = 13 u
Mass of Tank T 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 = 6500

1 u = 6500 ÷ 25 = 260

Mass of Tank S and the water in it more than mass of Tank T and the water in it 
= 13 u - 12 u
= 1 u

Fraction of the water that Tank S had more than Tank T 
= ¹₉ - ¹₁₀
= ¹⁰₉₀ - ⁹₉₀
= ¹₉₀ 

¹₉₀ of the water capacity → 1 u
⁹₉₀ of the water capacity → 9 x 1 u = 9 u

Mass of an empty tank
= 12 u - 9 u
= 3 u
= 3 x 260
= 780 g

Answer(s): 780 g