math

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if i drive home with a speed of 60km/h,I will arrive an hour ealier than expected.If i drive 40km/h then i will arrive an hour later than expected.At what speed should i travel in order to arrive BANG on time?

• math -

time to go home at 40 km/h --- d/40 hours
time to go home at 60 km/h --- d/60 hours

d/40 - d/60 = 2 (there is a 2 hr difference in the two times)
time 120
3d - 2d = 240
d = 240

so the two times are 240/40 and 240/60
or 6 and 4 hours,
so he wants to take 5 hours
speed = 240/5 = 48 km/h

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