Two identical tanks B and C contained some water. Tank B was ¹₂ filled and Tank C was ¹₃ 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 7 : 6. The total mass of the two tanks and the water they contained was 2340 g. What is the mass of an empty tank?
Mass of Tank B and the water inside = 7 u
Mass of Tank C and the water inside = 6 u

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

13 u = 2340

1 u = 2340 ÷ 13 = 180

Mass of Tank B and the water in it more than mass of Tank C and the water in it 
= 7 u - 6 u
= 1 u

Fraction of the water that Tank B had more than Tank C 
= 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 tank
= 6 u - 2 u
= 4 u
= 4 x 180
= 720 g

Answer(s): 720 g