Your math professor shows you 100 coins placed on his desk with their heads up. He says to you:

"Each time you choose exactly 93 coins and flip them, turning heads to tails or vice versa."

What is the least number of times that you need to do this in order to get all 100 coins tails up?

• Instead of flipping 93 coins at a time, look at it as "not flipping" 7 coins at a time. Now look at it as flipping 7 coins at a time. Obviously, one has to perform an even number of flips (as 93 is odd, 100 is even)
  One can flip 7 consecutive coins, 13 times, to get 91 Tails. (13) Now flip 86-91 and 92nd coins to get 86 Tails. (1) Now flip 86-91 and 93, and then 94-100 to get all Tails (2)
  Hence 16 tries.
• 10 years agoHelpfull: Yes(1) No(0)