An experiment consists of tossing a single die and observing the number of dots that show on the upper face. events a, b, and c are defined as follows.
a: observe a number less than 4.
b: observe a number less than or equal to 2.
c: observe a number greater than 3.

A single die is consists of 6 sides. The sides consists of one to six dots. Therefore, tossing a single die will have 6 equal probabilities. That is, one to six dots. 

(A) There are 3 numbers from 1 to 6 that are less than 4. Those are 1, 2, and 3. So, there are 3 possible outcomes. So, the probability of the event A happening is equal to 3/6 or 1/2. 

(B) There are 2 numbers less than or equal to 2. Those are 1 and 2, itself. The probability of this event happening is 2/6 or 1/3.  

(C) There are 3 numbers from 1 to 6 that are greater than 3. Those numbers are 4, 5, and 6. The probability of this event is equal to 3/6 or 1/2.