Two identical containers P and Q contained some water. Container P was ¹₂ filled and Container Q was ¹₃ filled. The ratio of the total mass of Container P and the water in it to the total mass of Container Q and the water in it was 5 : 4. The total mass of the two containers and the water they contained was 900 g. What is the mass of an empty container?
Mass of Container P and the water inside = 5 u
Mass of Container Q and the water inside = 4 u

Total mass of the two containers and the water inside them
= 5 u + 4 u
= 9 u

9 u = 900

1 u = 900 ÷ 9 = 100

Mass of Container P and the water in it more than mass of Container Q and the water in it 
= 5 u - 4 u
= 1 u

Fraction of the water that Container P had more than Container Q 
= ¹₂ - ¹₃
= ³₆ - ²₆
= ¹₆ 

¹₆ of the water capacity → 1 u
²₆ of the water capacity → 2 x 1 u = 2 u

Mass of an empty container
= 4 u - 2 u
= 2 u
= 2 x 100
= 200 g

Answer(s): 200 g