A box filled with 12 watermelons has a mass of 7.45 kg. The same box when filled with 6 lemons has a mass of 0.97 kg. The mass of 1 watermelon is five 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

7.45 kg = 7450 g
0.97 kg = 970 g

1 lemon = 1 u
6 lemons = 6 x 1 u = 6 u

1 box + 6 lemons = 970
1 box + 6 u = 970 --- (1)

1 watermelon = 5 u
12 watermelons = 12 x 5 u = 60 u

1 box + 12 watermelons = 7450
1 box + 60 u = 7450 --- (2)

(2) - (1)
60 u - 6 u = 7450 - 970
54 u = 6480

1 u = 6480 ÷ 54 = 120

6 u = 6 x 120 = 720

From (1):
1 box + 6 u = 970
1 box + 720 = 970
 
1 box = 970 - 720 = 250

Mass of 1 box = 250 g

Answer(s): 250 g