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.

What the heck i didn’t know

There is 240 red cars and there is 400 blue cars because 240/3=80 and 80*5=400