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 :
[tex]\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[/tex]
Justin's average speed to his parents' house: x= 40mph
Justin's average speed from his parents' house: x+20=60 mph
