Two identical containers Q and R contained some water. Container Q was ¹₂ filled and Container R was ¹₃ filled. The ratio of the total mass of Container Q and the water in it to the total mass of Container R and the water in it was 8 : 7. The total mass of the two containers and the water they contained was 3600 g. What is the mass of an empty container?
Mass of Container Q and the water inside = 8 u
Mass of Container R and the water inside = 7 u

Total mass of the two containers and the water inside them
= 8 u + 7 u
= 15 u

15 u = 3600

1 u = 3600 ÷ 15 = 240

Mass of Container Q and the water in it more than mass of Container R and the water in it 
= 8 u - 7 u
= 1 u

Fraction of the water that Container Q had more than Container R 
= 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
= 7 u - 2 u
= 5 u
= 5 x 240
= 1200 g

Answer(s): 1200 g