Question 804535
one walks at 3 mph
so at given time he is 3t miles away from starting point

Similarly

the other is 2t miles away

they are walking at right angles to each other

At a given time t they form a right angle with the starting point

Use Pythagoras theorem
(3t)^2+(2t)^2= 20^2

9t^2+4t^2=400

13t^2=400
t^2=400/13
t^2= 30.77
t= 5.55 minutes

say 6 minutes