A box filled with 14 durians has a mass of 16.87 kg. The same box when filled with 6 apples has a mass of 1.23 kg. The mass of 1 durian is seven times the mass of 1 apple. Find the mass of the box when it is empty. Express the answer in g.
1 kg = 1000 g

16.87 kg = 16870 g
1.23 kg = 1230 g

1 apple = 1 u
6 apples = 6 x 1 u = 6 u

1 box + 6 apples = 1230
1 box + 6 u = 1230 --- (1)

1 durian = 7 u
14 durians = 14 x 7 u = 98 u

1 box + 14 durians = 16870
1 box + 98 u = 16870 --- (2)

(2) - (1)
98 u - 6 u = 16870 - 1230
92 u = 15640

1 u = 15640 ÷ 92 = 170

6 u = 6 x 170 = 1020

From (1):
1 box + 6 u = 1230
1 box + 1020 = 1230
 
1 box = 1230 - 1020 = 210

Mass of 1 box = 210 g

Answer(s): 210 g