Answer:
Kristen can ride the bike for 20 hours
Explanation:
20 - 15 = 5
The rate is 0.25 an hour.
0.25 x 4 = 1 - Currently $4
0.25 x 4 = 1 - Currently $3
0.25 x 4 = 1 - Currently $2
0.25 x 4 = 1 - Currently $1
0.25 x 4 = 1 - No more
Add up all the 4's
She can ride the bike for 20 hours.