The probability would be split for this problem cause knowing with 20 coins , you can split that even amount ... here's what I would do..
Michelle has 20 coins , but only half of them land on heads and others on tails
As for a half , halves are always equaling to 1/2 or 50 (%) knowing we have 20 , you can evenly split that number of coins in a equal position
So ask your self .. what is half of 20? hmmm .. oh I got it , its 10! so knowing you have 20 but then later you split into half which is 10 ... meaning and knowing you have a 50% probability of getting the 20 coins hitting onto tails
hope this helps!