If they start out in the same place then the answer is a hundred miles (twice a 345 triangle's hypotenuse *10 miles). If they start X miles apart then we get to demonstrate some algebra :0)
I never bothered with memorizing the 345 special case rules. Pythagorean theorem (z = sqrt(x^2 + y^2)) works in all cases and yields the correct answer. Then again, public school, since it's so great, should've been able to teach me to memorize all the special cases. Yeah right.