Question 1151193
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The solution method shown by tutor @josgarithmetic is a perfectly good standard algebraic method.<br>
If you are one of many who dislike working with fractions, here is a very different, somewhat informal method.<br>
The ratio of the two speeds is 40:30 = 4:3; since the distances are the same, the ratio of the times is 3:4.<br>
Given that ratio of times, let the two times be 3x and 4x.  The difference, 4x-3x=x, is 20 minutes.<br>
That means the amount of time at 40mph was 3x = 3(20 minutes) = 60 minutes = 1 hour.  And 40mph for 1 hour makes the distance 40 miles.<br>