Hello Lennalyn,
The experimental probability of an event (also called the empirical probability) is found on the basis of observation. That is, the "probability experiment" is actually repeated a number of times (under conditions that are as identical as possible), and we then count the number of times the event of interest occurs. The experimental probability is basically the same as the relative frequency of the event. Let n be the number of trials (the number of times you repeat the experiment.) Let f be the frequency of the event of interest (number of times the event or outcome you wish to observe occurs.) Then the experimental probability of the event of interest is simply
P = f/n.
For the data you give, the experimental probability of observing heads is
P = 5/8 = .625
Hope that helps!
William
