An old man, with four sons and three daughters, buried a safebox with valuables inside. He wanted his children to get it when he died, but neither the boys nor the girls to get it all for themselves; he desired that at least two sons and two daughters had to be involved in order to find the missing treasure.
(For example, the three girls on their own couldn't find the treasure, even if one boy helped them. The four boys and one girl couldn't find it either.)
How could he manage this?
Bury the key in a different location that the safebox. Make 2 Maps. The first map (A) would give the location of the safebox, and the second map (B) would give the location of the key. Rip the the first map (A) into 4 pieces and give it to the sons. Rip the second map (B) into 3 pieces and give it to the daughters. All 7 sibling will have to be involved to find the contents of the box... otherwise, the sons are just left with a box, and the daughters have nothing more than a key.
*shrug*
Cheers, Jane