Question 1145798
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Let x be the average rate from A to B, in miles per hour. 

Then the average rate from B to C is  (x+30) mph.



The traveling time from A to B is  {{{207.5/x}}} hours;

The traveling time from B to C is  {{{220/(x+30)}}} hours.



The total time is 12.3 hours, which gives you an equation


    {{{207.5/x}}} + {{{220/(x+30)}}}  = 12.3  hours.


It is the "time" equation, and as soon as you get it, the setup is just completed.



To solve the equation, multiply both sides by x*(x+30)*10.  You will get


    2075*(x+30) + 2200*x = 123x*(x+1)


Simplify this quadratic equation and write it in the standard form


Then solve it and find its positive root.


It will be the answer for the average speed from A to B.


The average speed from B to C is 30 mph greater.
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I leave it to you to complete the solution.