Explanation:
Let s represent the number of small boxes and l represent the number of large boxes.
We have 2s = l and 150s + 450l = 4200.
Therefore 150s + 450(2s) = 4200,
150s + 900s = 4200, 1050s = 4200, s = 4.
l = 2s = 2(4) = 8.
Hence there are 4 small boxes and 8 large boxes.