問maths.....

Two cars A and B are travelling, A from the West, B from the North, which are perpendicular to each other, towards junction O, in the middle.

at the moment, cars A and B are 500 m and 200 m respectively from the junction O and the speeds of A and B are 40 km/h and 30 km/h.

find
(a) the time, in seconds, when they are closest to each other.
(b) the shortest distance (in metre) between two cars