If you have a stack of pennies without counting them how do you know if there is an even or odd number of them

If I have a stack of pennies. And I have to tell that without counting whether there are even number of pennies or odd.

Even numbers are the numbers which are divisible by 2 and odd numbers are the numbers which are not divisible by 2.

Then I will put the given pennies in 2 rows and then I will match them to form a pair of 2 pennies. After matching, if there is 1 penny left over, then there is an odd number of pennies and if all the pennies have a match,then there is an even number of pennies.

answer:

a=9

step-by-step explanation: lmk if you need a explanation