The probability of an event is equal to the number of favorable cases divided by the number of possible cases.

For example I do your question a).

a) Calculate the probability of getTing at least one die showing a 5 using two dices?

You can make a table to show all the possible and favorable cases. A event will be a par like

, or

for example. As it is a 6x6 table, there are

possible cases. And if you count the favorable cases, you realize that there are

of them. So applying the simple formula I gave you, there is a probability of

to get at least a 5 pitching once two dices.