Question 1167511
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You should know how to solve the problem using formal algebra, as shown by the other tutor.<br>
However, if a formal algebraic solution is not required, and/or if you want to get some good practice in logical thinking (and thus give your brain some good exercise), this problem lends itself to a quick and easy solution.<br>
The total trip was 10 hours; she stopped for a rest for 2 hours, so the total driving time was 8 hours.<br>
So her average speed was 400/8 = 50 mph.<br>
She drove for 4 hours at her original speed, so she drove for 4 hours at the reduced speed.<br>
Since she drove equal amounts of time at the two speeds, her overall average of 50 mph was halfway between her two driving speeds.<br>
Since the difference between the two speeds was 20 mph, an overall average of 50 mph means the two speeds were 50+10=60 and 50-10=40 mph.<br>
ANSWER: Her original speed was 60 mph.<br>
CHECK:
4 hours at 60 mph, plus 4 hours at 40 mph, equals 400 miles
4(60)+4(40) = 240+160 = 400<br>