A box filled with 10 watermelons has a mass of 8.26 kg. The same box when filled with 7 starfruits has a mass of 1.66 kg. The mass of 1 watermelon is four times the mass of 1 starfruit. Find the mass of the box when it is empty. Express the answer in g.
1 kg = 1000 g

8.26 kg = 8260 g
1.66 kg = 1660 g

1 starfruit = 1 u
7 starfruits = 7 x 1 u = 7 u

1 box + 7 starfruits = 1660
1 box + 7 u = 1660 --- (1)

1 watermelon = 4 u
10 watermelons = 10 x 4 u = 40 u

1 box + 10 watermelons = 8260
1 box + 40 u = 8260 --- (2)

(2) - (1)
40 u - 7 u = 8260 - 1660
33 u = 6600

1 u = 6600 ÷ 33 = 200

7 u = 7 x 200 = 1400

From (1):
1 box + 7 u = 1660
1 box + 1400 = 1660
 
1 box = 1660 - 1400 = 260

Mass of 1 box = 260 g

Answer(s): 260 g