SOLUTION: If it took Billy an hour to race over to his girlfriend’s house but two hours if he went 20 mph slower, how far away does Billy’s girlfriend live?

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Question 1079833: If it took Billy an hour to race over to his girlfriend’s house but two hours if he went 20 mph slower, how far away does Billy’s girlfriend live?
Answer by josmiceli(19441) About Me  (Show Source):
You can put this solution on YOUR website!
Let +d+ = the distance to his girlfriend's house
Let +s+ = his running speed in mi/hr
(1) +d+=+s%2A1+
(2) +d+=+%28+s-20+%29%2A2+
-----------------------
Plug (1) into (2)
(2) +s+=+%28+s-20+%29%2A2+
(2) +s+=+2s+-+40+
(2) +s+=+40+
-----------------------
Plug this result into (1)
(1) +d+=+40%2A1+
(1) +d+=+40+
His girlfriend lives 40 mi away
----------------------------
check:
(2) +d+=+%28+s-20+%29%2A2+
(2) +d+=+%28+40-20+%29%2A2+
(2) +d+=+20%2A2+
(2) +d+=+40+ mi
OK