Two identical tanks M and N contained some water. Tank M was ¹₂ filled and Tank N was ¹₃ filled. The ratio of the total mass of Tank M and the water in it to the total mass of Tank N and the water in it was 8 : 7. The total mass of the two tanks and the water they contained was 3900 g. What is the mass of an empty tank?
Mass of Tank M and the water inside = 8 u
Mass of Tank N and the water inside = 7 u

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

15 u = 3900

1 u = 3900 ÷ 15 = 260

Mass of Tank M and the water in it more than mass of Tank N and the water in it 
= 8 u - 7 u
= 1 u

Fraction of the water that Tank M had more than Tank N 
= 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
= 7 u - 2 u
= 5 u
= 5 x 260
= 1300 g

Answer(s): 1300 g