Given,
8 large boxes and 3 small boxes have a total weight of 142 kg
2 large boxes and 5 small boxes have a total weight of 61 kg.
Let us assume that the weight of 1 large box is x.
And the weight of 1 small box is y.
Thus the equation for the given problem can be written as
[tex]\begin{gathered} 8x+3y=142\text{ }\rightarrow\text{ (i)} \\ 2x+5y=61\text{ }\rightarrow\text{ (i}i) \end{gathered}[/tex]On multiplying the equation (ii) by 4,
[tex]8x+20y=244\text{ }\rightarrow\text{ (i}ii)[/tex]On subtracting equation (i) from equation (ii),
[tex]\begin{gathered} 8x-8x+20y-3y=244-142 \\ \Rightarrow17y=102 \\ \Rightarrow y=6\text{ kg} \end{gathered}[/tex]Thus the weight of one small box is 6 kg
On substituting the values of y in equation (ii)
[tex]\begin{gathered} 2x+5\times6=61 \\ \Rightarrow2x=61-30 \\ \Rightarrow x=\frac{31}{2} \\ \Rightarrow x=15.5\text{ kg} \end{gathered}[/tex]Thus the weight of one large box is 15.5 kg