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?