probability = # ways what you want happens / # ways everything happens
# of ways you can get heads exactly once- "4 choose 1" or 4! / (3!*1!) = 4
# of ways you can flip a coin 4 times = 2^4
4 / 2^4
4 / 16
1/4
P(exactly 1 heads when flipping a fair coin 4 times) = 1/4