Was a difficult puzzle – I got three correct answers for this. Vivek Shah, Mihir and Aman Singla in that order sent the correct answers – very impressed!
Here is the solution:
Make 2 stacks – one of 10 and the other of 90. 10 will have x heads and 90 will have 10-x heads.
If all the coins of the stack of 10 are reversed, both should have equal number of heads (10-x). However, this will increase the number of coins with head up.
Hope you enjoyed the puzzle!