<PRE><B>Question 312305</B>
The average speed for the first 45 miles was 15 mph and the average speed for the next 50 miles was 25 mph.
 What should be the speed for the next 85 miles so that the overall average speed would be 18 mph?
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Find the total distance: 45 + 50 + 85 = 180 mi
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Let s = average speed for 85 mi
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write a time equation: time = dist/speed
:
45mph time + 50mph time + s time = total time
{{{45/15}}} + {{{50/25}}} + {{{85/s}}} = {{{180/18}}}
3 + 2 + {{{85/s}}} = 10
5 + {{{85/s}}} = 10
{{{85/s}}} = 10 - 5
{{{85/s}}} = 5
Multiply both sides by s
5s + 85
s = {{{85/5}}}
s = 17 mph for 85 mi to average 18 mph
:
:
Check solution by finding the times
3 + 2 + 5 = 10</PRE>