Problem #195
Flip 'em all
Consider a row of N cards all facing up. Now a player flips all the multiples of 1, then 2 and so on till N. Find the number of cards facing up if N = 12345678987654321.
Consider a row of N cards all facing up. Now a player flips all the multiples of 1, then 2 and so on till N. Find the number of cards facing up if N = 12345678987654321.