It took Jen, who biked at a speed of 12 mph, 20 minutes to get home. What is Jim's speed, if it took him only 15 minutes to bike that distance?

You could find the distance first

v = 12 mph

t = 20 minutes = 20/60 = 1/3 of an hour

d = r * t

d = 12 * 1/3 = 4 miles.

And put that distance into Jim's time

d = 4 miles

r = ?

t = 15 minutes = 15/60 = 1/4 hour

r = d/t

r = 4/0.25 = 16 mph. I wish I could bike that quickly.

There is another way to do it more directly. The distance is the same for both of them.

r*t = R*T

R Jim's speed and r is jen's speed.

12 * 1/3 = R * 1/4 Multiply both sides by 4

12 * 1/3 * 4 = R Cancel 12 and 1/3

R = 16.