Question

Jean runs 8 mi and then rides 11 mi on her bicycle on a biathlon. She rides 9 mph faster than she runs. If the total time for her to complete the race is 1.5 hr, determines her speed running and her speed riding her bicycle.

Answer 1

Let Jean's speed while running = x mph

Then Jean's speed while riding will be = x+9 mph

Distance covered by running = 8 miles

Distance covered by riding = 11 miles

Total time taken to complete the race = 1.5 hours

As, $time=\frac{distance}{speed}$

So, time for running= $\frac{8}{x}$

And time for riding= $\frac{11}{x+9}$

Equation becomes:

$\frac{8}{x}+\frac{11}{x+9}=1.5$

Now, multiply every term by 2x(x+9) to clear denominators:  

$16(x+9)+22x=3x(x+9)$

Simplifying it we get

$3x^{2}-11x-144=0$

Solve the quadratic equation using formula

$x= \frac{-b+\sqrt{b^{2}-4ac }}{2a}$

putting a=3 , b= -11, c= -144

we get (x - 9)(3x + 16) = 0

where x=9 and x=$\frac{-16}{3}$

Neglect the negative answer as speed cannot be negative, so x = 9 mph

Hence, Jean's running speed is 9 mph and riding speed is x+9 = 9+9 = 18 mph