Question 1168915
Hi there,
Make the large boxes = x
Small boxes = y
12x + 2y = 199.........(1)
3x + 5y = 79...........(2)
Multiply (2) by 4
12x + 2y = 199........(1)
12x + 20y = 316.......(2)
Subtract equation (1) from (2)
18y = 117
y = 6.5 
Substitute y = 6.5 into equation (1)
12x + 2(6.5) = 199
12x + 13 = 199
12x = 199 - 13
12x = 186
x = 15.5
Large boxes weigh 15.5 kg
Small boxes weigh 6.5 kg.
Hope this helps :-)