Two identical tanks B and C contained some water. Tank B was ¹₈ filled and Tank C was ¹₉ filled. The ratio of the total mass of Tank B and the water in it to the total mass of Tank C and the water in it was 11 : 10. The total mass of the two tanks and the water they contained was 2100 g. What is the mass of an empty tank?
Mass of Tank B and the water inside = 11 u
Mass of Tank C 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 = 2100

1 u = 2100 ÷ 21 = 100

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

Fraction of the water that Tank B had more than Tank C 
= 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 100
= 200 g

Answer(s): 200 g