What is the formula for combinations

The product of 3 and a number increased by 8 is 31 written as an equation

Aiko jumped rope for 20 min and stopped at 8: 05 when did she start

The salesperson earns a 5% commission on the first $5,000 she has in sales. the salesperson earns a 7.5% commission on the amount of her sales that are greater than $5,000. this month the sales person had $8,000 in sales. what amount of commission, in dollars, did she earn?