a Use the formula for probability to find the number of favorable outcomes.
B
b We know the probability and the number of favorable outcomes. Use this to find the number of students in the class.
C
c The number of girls is the complement of the probability of the number of boys.
D
d How many boys are left of the total number of students when one is removed?
A
a 12
B
b 22
C
c P(Girl)=2/3
D
d P(Boy|Boy)=7/23
Practice makes perfect
a The probability of something is calculated by dividing the number of favorable outcomes with the number of possible outcomes.
P=Number of favorable outcomes/Number of possible outcomes
We know that the probability is 13 and the total number of outcomes is 36. By substituting this into the formula, we can solve for the number of favorable outcomes.
P=Number of favorable outcomes/Number of possible outcomes
As the number of favorable outcomes is 12, it means that the number of boys in the class is 12.
b If there are 11 boys in the class and we know the probability of picking a boy is 13, we can substitute this into the Probability Formula to find the total number of students in the class.
P=Number of favorable outcomes/Number of possible outcomes
The total number of students in the class is 33. If we subtract the number of boys from this number, we can determine the number of girls.
33-11=22
c Assuming there are only two genders in the class, the probability for choosing a girl will then be the complement of the probability of choosing a boy.
d Here, we know that the probability of selecting a favorable outcome is 13 and the total number of outcomes are 24. We can then calculate the number of favorable outcomes by substituting this into the formula.
P=Number of favorable outcomes/Number of possible outcomes
There are 8 boys in the class. After the teacher selects a boy, there are 7 boys left of 23 students. We then get the following probability of selecting a second boy.
P(Boy|Boy)=7/23
