Set up two equations:
5L + 6S = 175
3L + 2S = 83
Multiply the second equation by -3:
-9L -6S = -249
Now add the first and 3rd equations together:
5L + 6S = 175 + -9L -6s = -249
-4L = -74
Divide both sides by -4:
L = -74 / -4 = 18.5
Now replace L in the first equation and solve for s:
5(18.5) + 6S = 175
92.5 + 6S = 175
Subtract 92.5 from both sides:
6S = 82.5
Divide both sides by 6:
S = 82.5 / 6 = 13.75
Large box = 18.5 kilograms
Small box = 13.75 kilograms