How many 16-bit strings contain exactly nine 1's? I said the total number is 2 to the 16th power, so to get half with 1's would be 2 to the 8th, or 256. How many 16-bit strings contain at least fourteen 1's? I have no idea. When determining a how to get 2 pair in a poker hand, why is getting the denomination of the first pair, then the 2 cards, then the demonitation of the second pair, then the 2 cards wrong? I know you have to get the two denominations first, 13 choose 2 instead of 13 choose 1 and then 12 choose 1, but I don't know why.