A box filled with 13 durians has a mass of 16.61 kg. The same box when filled with 7 lemons has a mass of 1.49 kg. The mass of 1 durian is seven times the mass of 1 lemon. Find the mass of the box when it is empty. Express the answer in g.
1 kg = 1000 g

16.61 kg = 16610 g
1.49 kg = 1490 g

1 lemon = 1 u
7 lemons = 7 x 1 u = 7 u

1 box + 7 lemons = 1490
1 box + 7 u = 1490 --- (1)

1 durian = 7 u
13 durians = 13 x 7 u = 91 u

1 box + 13 durians = 16610
1 box + 91 u = 16610 --- (2)

(2) - (1)
91 u - 7 u = 16610 - 1490
84 u = 15120

1 u = 15120 ÷ 84 = 180

7 u = 7 x 180 = 1260

From (1):
1 box + 7 u = 1490
1 box + 1260 = 1490
 
1 box = 1490 - 1260 = 230

Mass of 1 box = 230 g

Answer(s): 230 g