in olden days people would sometimes say how smart they were (or someone else was) by saying 'I know how many beans make five!' The answer is usually thought to be five.
The answer I have heard to 'How many beans make five?' was:
A bean, a bean ,a bean and a half, half a bean and a bean.
Which , if you add it up, does come to five beans.
On the whole, though, saying s/he 'knows how many beans make five ' means that the person is worldly wise and not likely to be duped or taken advantage of.
2 beans, a bean and a half, half a bean and a bean has always been the answer that I have known ( and I am definately one of the older generation) it was just a phrase to catch the children out and they were always pleased to hear the answer but 'really' pleased when they could actually work it out.
Our answer was always, "one in my pocket and two in each hand"
Same sort of wuestion as "How long is a pice of string?" - answer - "twice its length when doubled"
My pet hate when I was a kid was my Dad's saying "How Hi is a Chinaman" and no matter what height I answered would always come back with "How Lo is his brother". I never got the joke, just got really frustrated.