Probability of an event happening = ____________.

Experimental probability is derived from situations, where an event is associated with an experiment that is recurring. It is also known as empirical probability. 

Experimental or empirical probability: P(E) =Number of trials where the event occurred/Total Number of Trials or, 

$\mathrm{Probability} = \frac{\mathrm{Total} \mathrm{number} \mathrm{of} \mathrm{favourable} \mathrm{outcomes} }{\mathrm{Total} \mathrm{number} \mathrm{of} \mathrm{possible} \mathrm{outcomes}}$.