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My equation does not work?

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?

Select an answer:
A. 17 hrs
B. 25 hrs
C. 31 hrs
D. 45 hrs

Answer:
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) :confused:

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?