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suppose you have 40 blue socks and 40 brown socks in a drawer.how many socks must you take from the drawer (without looking)
to be sure of getting a pair with the same color. a pair with different colors
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(a) to get a pair with the same color: 3 socks.
After getting any 3 socks, you inevitably will have a pair with the same color.
(b) to get a pair with different color: 41 socks.
In the worst case, your first 40 socks will be all of one color,
but the 41-th sock will be of the different color.