So here is how to solve the problem.
Let us analyze it first.
S + A = 70
S + L = 80
L + A = 80
Since S + L is equal to L + A, this would mean that S = A.
So S + A = 70 So A = 70/2 = 35kg
And lastly L. L= 80 -35 = 45kgs.
Therefore, Liam's weight is 45kgs.
Hope this explanation helps.
Let me know if you need more help next time. Have a great day!
