Question

Justin recently drove to visit his parents who live 96 miles away. On his way there his average speed was 20 miles per hour faster than on his way home (he ran into some bad weather). If Justin spent a total of 4 hours driving, find the two rates (in mph). Round your answer to two decimal places, if needed.
Justin's average speed to his parents' house: __________ mph

Justin's average speed from his parents' house: __________ mph

Answer 1

Answer:

Justin's average speed to his parents' house: x= 40mph

Justin's average speed from his parents' house: x+20=60 mph

Step-by-step explanation:

Let the average speed be x +20  

Then average speed from his parents' x

And the distance is S=96

Then we know :

$\displaystyle \frac{S}{x+20} +\frac{S}{x} =4 \\\\\\ \frac{96}{x+20} +\frac{96}{x}=4 \ |\div 4 \\\\\\ 24\bigg(\frac{1}{x+20} +\frac{1}{x} \bigg) =1 \\\\\\ 24\cdot \frac{x+x+20}{(x+20)x} =1 \ |\cdot (x+20)x \ \ \\\\\\48x+480=x^2+20x \\\\ x^2-28x-480=0 \\\\ \rm D=28^2+4\cdot 480=2704 =52^2 \\\\ \boxed{x_1=\frac{28+52}{2} =40} \\\\x_2=\frac{28-52}{2} =-12 \ \ \varnothing$

Justin's average speed to his parents' house:  x= 40mph

Justin's average speed from his parents' house:  x+20=60 mph