A farmer who owned 17 horses died and left the horses to his three sons. The will stipulated that the eldest son would receive half the horses, the middle son would receive a third of the horses, and the youngest son would receive one ninth of the horses.
Clearly there is a problem.
Can you see a solution using a little "creative" arithmetic?
The solution is to lend a horse to the sons, so now they have 18. The first son gets nine (a half), the second son gets six (a third) and the third son gets two (a ninth). Nine plus six plus two is 17, and they can return the loaned horse to you.
Yes, it is dodgy!
And if you can, what about another farmer who has horses and leaves one seventh of the horses to his youngest son, instead of one ninth.
Assuming the same creative solution works in this case, how many horses does the farmer have?
In this case
$$ 1/2 + 1/3 + 1/7 = 41/42 $$
So the farmer has 41 horses.