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 Rosyth Speed Qn. Pls help! Thanks in advance:)
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By: tancrowhurst (offline)  Saturday, August 11 2012 @ 03:14 AM CDT (Read 1607 times)  
tancrowhurst

David and his brother John cycled from home to the library using the same route, starting at the same time. David cycled at 15 km/h. When John covered 1/3 of the distance, David was 4.5 km ahead of him. David reached the library at 4.45 pm. What time did John reach the library?

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By: jo sarah (offline)  Saturday, August 11 2012 @ 10:13 AM CDT  
jo sarah

Since we know David's speed, he would take 4.5 km/15 km/h
= 0.3 h = 18 minutes to do 4.5 km.

So, when John did another 2/3 of the distance to arrive at the library,
David (if he continued) would be 2 x 4.5 km ahead,
i.e., he would take 2 x 18 minutes to do that distance.

So, John must arrive at the library at 36 minutes after 4.45 pm,
i.e., at 5.21 pm

[make sense? thanks]

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By: tancrowhurst (offline)  Saturday, August 11 2012 @ 08:10 PM CDT  
tancrowhurst

Thanks a lot, Jo Sarah! Smile

However, I think for 1/3 of the journey John travelled, David is 4.5 km ahead of him,
When John complete the journey, David would be 4.5 km x 3 = 13.5 km ahead of him.
Hence, John completes the journey 13.5/15 later than David.
John takes 0.9 h more to complete the journey.
He arrived at the library at 5.39 pm.

Thank you. Smile

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By: jo sarah (offline)  Monday, August 13 2012 @ 09:53 AM CDT  
jo sarah

Jee! You are right! Big Grin

I misread as David having arrived when John was 1/3 way.
thanks for clarifying! apologies for my mistake.

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By: tancrowhurst (offline)  Monday, August 13 2012 @ 10:24 AM CDT  
tancrowhurst

Thanks for your prompt Big Grin

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By: rgswathi (offline)  Wednesday, August 15 2012 @ 05:17 AM CDT  
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Quote by: tancrowhurst

Thanks for your prompt Big Grin

mm the best way

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