A student on the cross-country team runs 30 minutes a day as a part of her training. Write an equation to describe the relationship between the distance she runs in miles, D, and her running speed, in miles per hour when she runs:

a. at a constant speed of 6 miles per hour the first 15 minutes, and then at 5.5 miles per hour the last 10 minutes

b. at a constant speed of 5.4 miles per hour for m minutes, and then b miles per hour for n minutes