Fiora starts riding her bike at 20 mi/h. After a while, she slows down to 14 mi/h, and maintains that speed for the rest of the trip. The whole trip of 112 miles takes her 6.5 hours. For how long did she travel at 20 mi/h?

3.5 hours

Explanation:

Speed = distance/time

Let the distance that Fiora biked at 20 mi/h through be x miles and the time it took her to bike through that distance be t hours at 20 mi/h

Then, the rest of the distance that she biked at 14 mi/h is (112 - x) miles

And the time she spent biking at 14 mi/h the rest of the distance = (6.5 - t) hours

Her first biking speed = 20 mph = 20 miles/hour

Speed = distance/time

20 = x/t

x = 20 t (eqn 1)

Her second biking speed = 14 mph = 14 miles/hour

14 = (112 - x) / (6.5 - t)

112 - x = 14 (6.5 - t)

112 - x = 91 - 14t (eqn 2)

Substitute for x in (eqn 2)

112 - 20t = 91 - 14t

20t - 14t = 112 - 91

6t = 21

t = 3.5 hours

x = 20t = 20 * 3.5 = 70 miles.

(112 - x) = 112 - 70 = 42 miles

(6.5 - t) = 6.5 - 3.5 = 3 hours

Meaning that she travelled at 20 mi/h for 3.5 hours.