How can we find percent of a number

For each proportion find the unknown number x 1/5/8=×/15

An arc subtends a central angle measuring 7pi/4 radians what fraction of the circumference is this arc?

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?