Question 1208018
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In the problem, speeds are in miles per hour and times are in minutes.<br>
In general, you can get into all kinds of trouble if you work a problem with mixed units.  It would be better mathematics if you change miles per hour to miles per minute, or change minutes to hours.<br>
However, in this simple problem, working with mixed units is okay.<br>
The way the problem is posed, you are apparently supposed to use the result from part (a) to find the answer to part (b).<br>
However, finding the answer to part (b) is trivial if you use the fact that, because of the inverse variation (using mixed units!), the product of the speed in miles per hour and the time in minutes is constant:<br>
30mph * 40 minutes = 40mph * x minutes<br>
30*40 = 40*x<br>
Trivially, the answer is 30 (minutes)<br>