Originally Posted by

**galactus** Think of ol' Pythagoras.

The distance between the cars at the time they are .5 and .7 from the intersection, respectively, can be found from

$\displaystyle D=\sqrt{.5^{2}+.7^{2}}=.86 \;\ km$

Just differentiate implicitly w.r.t time, $\displaystyle D^{2}=x^{2}+y^{2}$

$\displaystyle D\frac{dD}{dt}=x\frac{dx}{dt}+y\frac{dy}{dt}$

$\displaystyle \frac{dD}{dt}=\frac{x\frac{dx}{dt}+y\frac{dy}{dt}} {D}$

Plug in your knowns on the right side (they are all given) and solve for dD/dt