Isla flipped a coin 30 times. the coin landed heads up 9 times and tails up 21 times.

what is the theoretical probability of the coin landing heads up? show your work.

Let's start with the fraction, 9/30. By the theoretical probability I'm assuming it means a percentage, so to turn a fraction into a percentage we divide the top by the bottom (9÷30) and multiply by 100. 30% 

Or 9/21, and that would be 42.86%.

It depends on if it wants a basic probability on how many times it will land on heads, if it does use 42.86%. If it wants a probability out of 30 it's 30%. The question isn't very clear.