Question 1088680
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Let r be her running speed, in miles per hour.

Then her biking speed is (r+18) mph.


The time of running 3 miles was {{{3/r}}} hours. (= distance/rate)

The time of biking was {{{12/(r+18)}}} hours.    (= distance/rate)


The condition says that 

{{{3/r}}} = {{{12/(r+18)}}}.


It is your equation. You must solve it to find the unknown r.


I can solve it mentally: r = 6 mph.


If you want to solve it analytically, then multiply both sides by r*(r+18); then simplify. You will get a quadratic equation.


Solve it by factoring or use the quadratic formula.


You just know the answer.
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