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
