Here's an old challenge from an old book (I think it goes like this, if I remember well):
There is a place on Mississippi river between Missouri and Kentucky where are two ferry boats: the vintage Old Town and the modern president. One time, exactly at the same time, the Old Town left the Missouri shore and the president left the Kentucky shore.
Each ferry traveled at a fixed velocity and, after a while, they intersected each other 9000 feet from Missouri. When they arrived to the other side, they didn't waste any time and immediately started the journey back to intersect each other 3000 feet from Kentucky.
What is the width of the river on the site where this story takes place?
How far away from Kentucky will they intersect for the third time?
EDIT: For those who voted to close this question, this is NOT a mathematics problem. It's a mathematics puzzle. At least it's from an old book of puzzles (which title I honesty can't remember). You can't solve it by the use of conventional formulas like velocity or others. Can you at least leave a comment?