Question 81480
When Allison leaves, 2 hours later, Sheila will be already be 60 miles from the start point (ie 2 hours at 30mph).
At this moment, just as Allison puts her car in gear and floors the throttle, let's start the stopwatch and call it time t=0.
Now let us say that the two of them meet up at time t = T.
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Now, distance = speed x time, so when they meet up at time = T, Allison will have travelled 60T miles.
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In that time period, the more sedate Sheila will have travelled 30T miles. Add this to the 60 miles head-start she got (see first paragraph)and the total distance she has traveled since setting off is (30T + 60) miles.
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Now the clever, bit.  At the point they meet up, they must both be exactly the same place which implies they are both the same distance from the start. So we can put our two equations for T equal to each other
60T = 30T + 60
30T = 60
T= 2 hours  (ANSWER)