Conditional Probability

http://www.mathsisfun.com/data/probability-events-conditional.html

I have always found probability to be a tricky subject.. A tree diagram helps in dealing with more than one event but in  a single event, things can be different as  can be seen in one of the solutions from Khan Academy, World of Maths below:-

 Let A be the event that the first die is 1 and B be the event that the dice sum to 3. We can find P(B | A) by finding all of the outcomes where the sum of the dice is 3, given that the first die is 1. There is only 1 outcome that meets these conditions, and there are 6 total outcomes where the first die is 1,

Therefore, P(B | A)=16.

The video below appears to be dealing with more than one event.

    Calculating conditional probability: