How do you solve probability

Step-by-step explanation:

probability=number of times of outcome/total number of possible outcomes

for example;

in a class of 12 boys, and 18 girls ,the probability that a student selected from the class is a boy is

p(B)=12/30=2/5, since the number of boys in the class is 12 and the total number of students in the class is 30 also the probability that a girl is picked

=18/30=3/5.

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what chart am i supposed to follow?