Show that if there are 15 points scattered inside a 1 unit square, then at least 3 of them can be covered by a square of side $1/5$ I have been trying to find a proper way to divide the square into smaller squares but nothing seems to be working so far, that is, trying to apply in a straight forward way the pidgeonhole principle. I tried dividing the square into rectangles but that didn't seem to work either (14 rectangles of sides $1/7\times 1/2$). Could this be solved by contradiction? Any suggestions?

I'm sorry, but I simply don't see it:

or better:

