Question 379149
In 1/2 hour the 1st plane is 250 miles away (500*0.5)
The 2nd plane gains on the 1st at 100 mph (600 - 500)
250/100 = 2.5 hours
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The other tutor did it the "hard way", which increases the chance of error.
The 3 hours she found is the time the 1st plane flew, not the time for the 2nd to overtake the 1st.