Question 186413
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Okay, sorry for this but let us re-state the problem here to make it clear.
At 5:00pm, Adam and David were on Eastbound highway and Adam, travelling at {{{60mi/hr}}} was 10 miles <font color="red">ahead</font> of David, right? (east of David, meaning farther east?)
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The question is, when David accelerated to {{{70mi/hr}}}, at what TIME did he passed Adam who's travelling at {{{60mi/hr}}}?
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*Remember: {{{Speed=Distance/Time}}}--->{{{S=D/T}}}, and {{{D=S*T}}}
*"D" is constant since no one stopped--->{{{D[A]=D[D]}}}
{{{S[A]*T+highlight(10mi/hr)=S[D]*T}}}, remember to add 10mi/hr because he's ahead.
{{{(60mi/hr)(T)+10(mi/hr)=70(mi/hr)T}}}
{{{10=70T-60T}}}
{{{10=10T}}}=====> {{{cross(10)1/cross(10)1=cross(10)T/cross(10)}}}
{{{highlight(T=1hour)}}}, after 1 hour they'll be at the same Distance.
Check:
{{{S[A]*T+10=60(mi/cross(hr))*1*cross(hr)+10=70miles}}}
{{{S[D]*T=70(mi/cross(hr))*1*cross(hr)=70miles}}}
So, <font color="red">after 1 hour</font>(pass 6:00pm), David passed Adam, for David is faster.
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Thank you,
Jojo</pre>