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# My equation does not work? Watch

1. Hi,

Two boys starting from the same place walk at a rate of 5kmph and 5.5kmph respectively. What time will they take to be 8.5km apart, if they walk in the same direction?

A. 17 hrs
B. 25 hrs
C. 31 hrs
D. 45 hrs

Option
A
Explanation:
The relative speed of
the boys = 5.5kmph – 5kmph
= 0.5 kmph
Distance between them is 8.5 km
Time= 8.5km / 0.5 kmph =
17 hrs

I understand the internet's solution but wonder why mine would not work:

Boy 1: Rate 5 kmph x t = 5t (distance)

Boy 2; Rate 5.5 kmph x t = 5.5t (distance)

Distance of Boy 1 + Distance of Boy 2 = 8.5 (apart)

5t + 5.5t = 8.5
10.5t = 8.5
t = 0.809 hr.

Obviously my solution is wrong but why? Is there a similar but correct that I could have used to mine?
2. It's not 5t + 5.5t = 8.5, it should be 5.5t - 5t = 8.5 since you're trying to find the difference, then you'd have the right answer.
3. (Original post by 0x2a)
It's not 5t + 5.5t = 8.5, it should be 5.5t - 5t = 8.5 since you're trying to find the difference, then you'd have the right answer.
Thanks, I guess it was obvious.

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Updated: April 9, 2013
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