In a biased coin probability of a head is P. In this coin, both head and tail cannot happen simultaneously since it is a dependent event. So I thought the answer is undefined.
But the Probability of an impossible event is zero(0).
So wheather the above problem is an impossible event or undefined event (Note: Undefined event you should not calculate probability)
You might call it a null event. Essentially an event is defined as a set (of outcomes). A null event corresponds to the empty set. The probability associated with the empty set is zero.
Since events are defined as sets you can't have "undefined" events.