strip diagram with algebra equtions

Question

There are 400 boxes arranged in 3 groups. The second group has twice as many boxes as the first group. The third group has 4 more boxes than the first group. How many boxes are in the first group? Solve this problem in two ways: with the aid of a strip diagram and with algebraic equations. Explain your reasoning.

Answer 1

S=2F, T=F+4

F+S+T = 400

F+2F + F+4 = 400

4F = 396

F = 99 boxes in first group

S= 198 boxes in 2nd group

T= 103 boxes in 3rd group