Each day a man meets his wife at the train station after work, and then she drives him home. She always arrives exactly on time to pick him up. One day he catches an earlier train and arrives at the station an hour early. He immediately begins walking home along the same route the wife drives. Eventually his wife sees him on her way to the station and drives him the rest of the way home. When they arrive home the man notices that they arrived 20 minutes earlier than usual. How much time did the man spend walking?
Lets say normally he get's in at 6 and they get home at 7. Today, he get's in at 5 and they get in at 6:40. However, because the man starts walking and moves closer to his home when his wife picks him up, the amount of time he spends in the car with her will not be an hour as usual, but less. He travels (walk+car) for a total of 100 minutes. If they arrive 20 minutes earlier than usual, that means the distance (and time) his wife covered is less because his walking reduced the distance between himself and home. His wife normally drives an hour. (as established, she picks him up at 6 and arrives at 7)...From here on, I am lost. How does the fact that they arrived 20 minutes earlier tell us that he walked for 50 minutes? If the question said that the round trip took 20 minutes less than normal then I could understand how each leg of the trip was reduced by 10 minutes, but the question says that it took them 20 minutes less time to get home from normal when the husband got picked up at his closer-to-home distance.
This problem is without question one of the hardest and most frustrating I have encountered. I don't understand why we care about the round trip of the wife. If it normally takes him and her one hour to get home and today they arrived 20 minutes earlier doesn't that apply just to the leg of the trip? I could see this, perhaps, if her trip was reduced by 20 minutes (i.e. the round trip) but its only the second half which the two of them both travel that is reduced by 20 minutes. This means that the round trip would be reduced by 40 minutes but again, how does this help us?
1. Let the wife drive for y minutes till the husband starts from the station.
2. Once the husband starts the let the wife drive for x minutes till she meets him. Thus the husband also walks for x minutes.
3. After meeting her husband the wife drives back for the same duration of x+y minutes
4. The wife totally traveled for 2x + 2y minutes.
5. We actually know that they arrived 20 minutes earlier.
6. If y=0. meaning that the wife had started at the same time as the husband i.e, say at 5 then it means she normally comes at 6 and hence takes 1 hr to travel to the station and totally 2 hrs to and fro. but that day she traveled 20 min less so the duration of the travel is 100 minutes
7. Equating (4) and (6), 2x+2y=100 or x=50 since y=0, we have x is 50 minutes or the man walked for 50 minutes
Note: whatever value we plug in for y, we will get x as only 50 because the difference between the RHS and 2y is always 100. y can also be negative i.e., the wife starts after the husband starts.
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