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# Probability of simple events

Probability is the chance that a certain event will happen. It is defined as the ratio of the number of ways a specific event -called a positive outcome - can happen to the total number of possible outcomes. That is:

$Probability\;=\;\frac{Number\;of\;positive\;outcomes}{Total\;number\;of\;possible\;outcomes}$

What is the probability that a day of the week chosen at random will begin with the letter S?

There are two positive outcomes: Saturday and Sunday.
There are seven days in a week, so there are seven total possible outcomes.
Using the rule above, you can see that the probability is

$Probability\;=\;\frac{Number\;of\;days\;beginning\;with\;the\;letter\;S}{Total\;number\;of\;days\;of\;a\;week}=\frac{2}{7}$

The probability that a day chosen at random will begin with the letter S is $\frac{2}{7}$

 A spinner has 4 equal sectors colored yellow, blue, green and red. After spinning the spinner, what is the probability of landing on each color?

The possible outcomes of this experiment are yellow, blue, green, and red.

$P(yellow)\;=\;\frac{Number\;of\;ways\;to\;land\;on\;yellow}{Total\;number\;of\;colors}=\frac{1}{4}$

$P(blue)\;=\;\frac{Number\;of\;ways\;to\;land\;on\;blue}{Total\;number\;of\;colors}=\frac{1}{4}$

$P(green)\;=\;\frac{Number\;of\;ways\;to\;land\;on\;green}{Total\;number\;of\;colors}=\frac{1}{4}$

$P(red)\;=\;\frac{Number\;of\;ways\;to\;land\;on\;red}{Total\;number\;of\;colors}=\frac{1}{4}$