When x < 100, y = X.

When x≥ 100, y=x-25.

The consumption function of Company B:

y=0.85x

Drawings available:

When X < 100, company b is cheap.

When 100 ≤ x < 500/3, Company A is cheap.

When x=500/3, the two companies are equally cheap.

When X > 500/3, Company B is cheap.