David M. answered • 08/26/19
Figuring out the patterns and learning to count
.Probability is desired compared to all possible. In-the case of the 8th grade,there are 3 boys compared to 10 (7 girls plus 3 boys) possible students, so the probability a boy is picked from 8th grade is 3 out of 10, written "3:10" or "3/10". There is an advantage to treating as a fraction "3/10" , so I will stick with that. So, the probability of picking a boy from 7th grade is, similarly, 5/10=1/2. Picking a boy from the 8th grade and picking a boy from the 7th grade are independent actions. Therefore, we Multiply the 2 probabilities together to find our answer: (3/10)x(1/2 ) = (3/20). If we were looking for the probability of choosing a girl from the 8th grade and a boy from the 7th grade, the answer would be (7/10)x(1/2)=(7/20). It is important to note that for the simple multiplication of the probabilities to work, each event cannot depend in any way on the other event. In this case, which student is chosen from the 7th grade does NOT depend on which student is chosen from the 8th grade, or visa versa.
