Box W and Box X are identical. ⁴₅ of Box W and ³₇ of Box X are filled with sand. The mass of Box X with its contents is ⁸₉ the mass of Box W with its contents. The total mass of the two boxes with their contents is 221 kg. Find the mass of an empty box.
Mass of Box W and its contents = 9 u
Mass of Box X and its contents = 8 u

Total mass of the two boxes with their contents
= 9 u + 8 u
= 17 u

17 u = 221 

1 u = 221 ÷ 17 = 13

Mass of Box W with its concents more than mass of Box X with its contents
= 9 u - 8 u
= 1 u
= 1 x 13
= 13 kg

Fraction of the sand that Box W had more than Box X
= ⁴₅ - ³₇
= ²⁸₃₅ - ¹⁵₃₅
= ¹³₃₅

¹³₃₅ of the capacity of sand in a box → 13
¹₃₅ of the capacity of sand in a box → 13 ÷ 13 = 1
¹⁵₃₅ of the capacity of sand in a box → 15 x 1 = 15 

Mass of the sand and Box X
= 8 u
= 8 x 13
= 104 kg

Mass of the empty box
= 104 - 15
= 89 kg

Answer(s): 89 kg