Question 813170
Average speed is always ( total distance traveled ) / ( total time spent traveling )
It is not an average of several average speeds
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1st part of trip:
{{{ 15 = 21t }}}
{{{ t = 15/21 }}}
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2nd part:
{{{ 20 = 18t }}}
{{{ t = 20/18 }}}
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3rd part:
{{{ 30 = 12t }}}
{{{ t = 30/12 }}}
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The total distance traveled is {{{ 15 + 20 + 30 = 65 }}} mi
The total time is {{{ 5/7 + 10/9  + 5/2 }}} hrs
You can find the common denominator, or just
use decimal equivilants
{{{ .71429 + 1.111 + 2.5 }}}
{{{ t = 4.3254 }}} hrs
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{{{ d/t = 65/4.3254 }}} mi/hr
{{{ d/t = 15.0275 }}}
The average speed is 15.0275 mi/hr
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You can check this