Answer:
 28 miles
Step-by-step explanation:
Let w stand for the number of miles Derrick runs each week. Then after 4 weeks of training, he wants the total mileage to be 100 miles.
 16 + 3w = 100
 3w = 84 . . . . . . . subtract 16
 w = 28 . . . . . . . . divide by 3
Derrick will run 28 miles each week to achieve his training goal.