If it would take Billy an hour to race over to his girlfriend's house and two hours if he went 20 mph slower, how far away does Billy's girlfriend live? ... its 40 miles lol I guessed

Let d (miles) = the distance to Billy's girlfriend's house.

Let v (mph) be Billy's speed when his time of travel is one hour.

Because distance = velocity * time,

therefore

time = distance/velocity.

Therefore

time = d/v = 1

v = d (1)

When Billy's speed is 20 mph slower, the time of travel is 2 hours.

Therefore, the time is

d / (v - 20) = 2

Multiply each side by (v - 20).

d = 2 (v - 20) (2)

Substitute (1) into (2).

d = 2 (d - 20)

d = 2d - 40

0 = d - 40

40 = d

Answer: 40 miles