When I walk from my house at 4km/h, I will get to the office 30 minutes later than when I walk at 5km/h. Calculate the distance between my house and office.

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?