bullfromist wrote:
why can't this problem be solved by the equation 60(t-15)=50t ?
You could use that equation, but if you did, then you would be solving for Mary's time, not Paul's time (remember that Mary had a 15-minute head start). Mary's time = 90 min (1.5 hours), subtract 15 to get Paul's time of 75 (1 hour, 15 minutes).
In general, it's best to solve for the variable asked for by the question, and to avoid unnecessary extra steps such as these.
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