Two identical containers D and E contained some water. Container D was ¹₃ filled and Container E was ¹₄ filled. The ratio of the total mass of Container D and the water in it to the total mass of Container E and the water in it was 7 : 6. The total mass of the two containers and the water they contained was 3900 g. What is the mass of an empty container?
Mass of Container D and the water inside = 7 u
Mass of Container E and the water inside = 6 u

Total mass of the two containers and the water inside them
= 7 u + 6 u
= 13 u

13 u = 3900

1 u = 3900 ÷ 13 = 300

Mass of Container D and the water in it more than mass of Container E and the water in it 
= 7 u - 6 u
= 1 u

Fraction of the water that Container D had more than Container E 
= ¹₃ - ¹₄
= ⁴₁₂ - ³₁₂
= ¹₁₂ 

¹₁₂ of the water capacity → 1 u
³₁₂ of the water capacity → 3 x 1 u = 3 u

Mass of an empty container
= 6 u - 3 u
= 3 u
= 3 x 300
= 900 g

Answer(s): 900 g