72. Determine the distance. Jones set out from $A$ to $B$ and on the way, 10 km from $A$, he met his friend Kenward, who had set out from $B$ at the same time as him. Upon reaching $B$, Jones immediately turned back. Kenward did the same upon reaching $A$. The friends met again, but this time 12 km from $B$. Of course, each walked at a constant speed. What is the distance between $A$ and $B$?

There is a simple rule that anyone can use to find the required distance mentally in a few seconds. If you know how to proceed, the problem is solved with unusual ease.
72. The distance between two points is 18 km. The meeting points are 10 km and 12 km away from \(A\) and \(B\) respectively. Multiply 10 (the first distance) by 3 and subtract the second distance -12. What could be simpler? Try other distances to the meeting points (making sure the first distance is more than \(\frac{2}{3}\) of the second) and you will find that this rule works with unfailing success.