Two identical tanks R and S contained some water. Tank R was ¹₂ filled and Tank S was ¹₃ filled. The ratio of the total mass of Tank R and the water in it to the total mass of Tank S and the water in it was 8 : 7. The total mass of the two tanks and the water they contained was 3750 g. What is the mass of an empty tank?
Mass of Tank R and the water inside = 8 u
Mass of Tank S 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 = 3750

1 u = 3750 ÷ 15 = 250

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

Fraction of the water that Tank R had more than Tank S 
= 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 250
= 1250 g

Answer(s): 1250 g