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Probability

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• Probability: the measure of an outcome to the total possible outcomes
• Probabilities are often in the form of a fraction
• The top of the fraction is the number of the desired outcome
• the bottom of the fraction is the total number of possible outcomes

There are 3 red marbles, 6 blue marbles, and 3 green marbles in a bag. If I reach in and grab one marble, what is the probability that the marble will be red?

• Total marbles = 3 + 6 + 3 = 12
• [red/total]
• [(3 ÷3)/(12 ÷3)] = [1/4]

There are 4 red marbles, 10 blue marbles, and 6 green marbles in a bag. If I reach in and grab one marble, what is the probability that the marble will be blue?

• Total = 4 + 10 + 6 = 20
• [blue/total] = [10/20]
• [(10 ÷10)/(20 ÷10)] = [1/2]

There are 12 boys and 8 girls in the room. If one student is chosen at random, what is the probability that the student will be a girl?

• Total students = 12 + 8 = 20
• [girls/total] = [8/20]
• [(8 ÷4)/(20 ÷4)]

There are 30 boys and 17 girls in the room. If one student is chosen at random, what is the probability that the student will be a girl?

• Total students = 30 + 17 = 47
• [girls/total] = [17/47]

There are 7 red pens, 11 blue pens, and 3 green pens in a box. If I reach in and grab one pen, what is the probability that the pen will be green?

• Total = 7 + 11 + 3 = 21
• [green/total] = [3/21]
• [(3 ÷3)/(21 ÷3)]

There are 9 red pens, 15 blue pens, 3 green pens, and 12 yellow pens in a box. If I reach in and grab one pen, what is the probability that the pen will be green?

• Total = 9 + 15 + 3 + 12 = 39
• [green/total] = [3/39]
• [(3 ÷3)/(39 ÷3)]

There are 13 red pens, 12 blue pens, 11 green pens, and 14 yellow pens in a box. If I reach in and grab one pen, what is the probability that the pen will be yellow?

• Total = 13 + 12 + 11 + 14 = 50
• [yellow/total] = [14/50]
• [(14 ÷2)/(50 ÷2)]

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*These practice questions are only helpful when you work on them offline on a piece of paper and then use the solution steps function to check your answer.

Probability

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