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

On multiplying the equation (ii) by 4,

On subtracting equation (i) from equation (ii),

Thus the weight of one small box is 6 kg
On substituting the values of y in equation (ii)

Thus the weight of one large box is 15.5 kg