Math percentage question

Question

Amandas job offered two plans, plan A is a salary of $1,200 per month plus 6% commission on sales. Plan B is a salary of $1,600 per month and 4% commission on sales. For what amount of monthly sales is plan A better then plan B?

Answer 1

1200+.06S = 1600+.04S

.02S = 400

S = 400/.02 = $20,000 in monthly sales makes Plan A and B equal

IF you sell > $20K monthly, Plan A is best