Originally Posted by

**Archie Meade** You haven't handled the division properly here, Mukilab

$\displaystyle Av=\frac{2D}{\frac{40}{D}+T_2}=\frac{2D}{\frac{40+ DT_2}{D}}=\frac{2D^2}{40+DT_2}$ etc

However, because we are dealing with average speeds, the situation is __linear__, so we can choose a distance of 40 miles for convenience.

$\displaystyle \frac{2D}{1+x}=60$

since the car will cover the 40 miles in 1 hour, so since D=40

$\displaystyle \frac{80}{1+x}=60$

$\displaystyle 80=(1+x)60=60+60x$

$\displaystyle 20=60x$

$\displaystyle x=\frac{1}{3}$ hours = 20 minutes.

On the return journey the car covers 40 miles in 20 minutes,

so what is it's average speed on the return journey?