This puzzle is similar to 10 Coins Puzzle. 
  
Question: 100 coins are lying flat on a table. 10 of them are heads up and 90 are tails up.You can’t see which one is which.How can we split the coins into two piles such that there are same number of heads up in each pile?

Note: It is allowed to flip the coins of one pile once.
  
Answer: Make 2 piles with 10 coins and 90 coins each. Now, flip all the coins in the smaller pile. 
  
Explanation : Make 2 piles and say pile one has ‘h’ no. of heads and ‘t’ no. of tails while the other pile will have ’10-h’ heads and ’90-t’ tails.Now we can flip coins of one pile,so lets flip coins of any pile(say 2nd pile) then no. of heads and tails interchange and now no. of heads are ’90-t’ and no. of tails are ’10-h’.

Now no. of heads in pile 1 = no. of heads in pile 2

h=90-t

Therefore,h+t=90

And no. of coins in pile 1 is ‘h+t’

So,split the coins into 90 and 10.
  
This article has been contributed by Vikash Kumar. 
  
Please write comments if you find anything incorrect, or you want to share more information about the topic discussed above
 you are allowed to flip the pile once?