Marcus can choose between a monthly salary
of $1,500 plus 5.5% of sales or $2,400 plus 3% of sales. he expects sales
between $5,000 and $10,000 a month. which salary option should he
choose? explain

Marcus should choose $2,400 plus 3% of sales. 

His salary is composed of a fixed salary and a variable salary.

Fixed Salary   +   Variable Salary  = Monthly Salary
2400                   150 (3% of 5k)    = 2350  - minimum salary he'll receive
2400                   300 (3% of 10k)  = 2700  - maximum salary he'll receive

VS

Fixed Salary    + Variable Salary  = Monthly Salary
1500                  275 (5.5% of 5k) = 1775 - minimum salary he'll receive
1500                  550 (5.5% of 10k) = 2,050 - maximum salary he'll receive

As you can see, despite the high percentage of sales offered, it is not a good option because the fixed salary is low as compared to the other option. Even the maximum salary he'll receive under the option $1500 is way lower than the minimum salary he'll receive under the option $2400.