Answer: Large Box: 18.75 kilograms
Small Box: 15.75 kilograms
Explanation:
l=large box. s=small box
5l+3s=141
7l+9s=273
(5l+3s=141)*3
15l+9s=423 ==> subtract this equation
7l+9s=273 ==> subtract this equation
8l=150 ==> subtract the two above equations
l=150/8
l=18.75
5(18.75)+3s=141
93.75+3s=141
93.75+3s=141.00
3s=47.25
s=15.75
Large Box: 18.75 kilograms
Small Box: 15.75 kilograms