Box J and Box K are identical. ⁵₆ of Box J and ³₅ of Box K are filled with sand. The mass of Box K with its contents is ⁹₁₀ the mass of Box J with its contents. The total mass of the two boxes with their contents is 665 kg. Find the mass of an empty box.
Mass of Box J and its contents = 10 u
Mass of Box K and its contents = 9 u

Total mass of the two boxes with their contents
= 10 u + 9 u
= 19 u

19 u = 665 

1 u = 665 ÷ 19 = 35

Mass of Box J with its concents more than mass of Box K with its contents
= 10 u - 9 u
= 1 u
= 1 x 35
= 35 kg

Fraction of the sand that Box J had more than Box K
= ⁵₆ - ³₅
= ²⁵₃₀ - ¹⁸₃₀
= ⁷₃₀

⁷₃₀ of the capacity of sand in a box → 35
¹₃₀ of the capacity of sand in a box → 35 ÷ 7 = 5
¹⁸₃₀ of the capacity of sand in a box → 18 x 5 = 90 

Mass of the sand and Box K
= 9 u
= 9 x 35
= 315 kg

Mass of the empty box
= 315 - 90
= 225 kg

Answer(s): 225 kg