Planning & Decision Making · Guided Practice
Calculating Basic Probabilities
8 questions with answers. Try each one before revealing the answer.
1
A bag contains red counters, blue counters and green counters.
One counter is chosen at random.
Calculate the probability that it is blue.
One counter is chosen at random.
Calculate the probability that it is blue.
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Answer
Total counters
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2
A bag contains red counters, blue counters and green counters.
One counter is chosen at random.
Calculate the probability that it is not blue.
One counter is chosen at random.
Calculate the probability that it is not blue.
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Answer
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3
In a class of pupils, cycle to school.
One pupil is chosen at random.
Calculate the probability that the pupil does not cycle to school.
One pupil is chosen at random.
Calculate the probability that the pupil does not cycle to school.
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Answer
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4
A fair six-sided die is rolled once.
State whether 'rolling a ' and 'rolling an even number' are mutually exclusive. Justify your answer.
State whether 'rolling a ' and 'rolling an even number' are mutually exclusive. Justify your answer.
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Answer
They are mutually exclusive.
is not an even number, so the two events cannot both happen on the same roll.
is not an even number, so the two events cannot both happen on the same roll.
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5
Two counters are taken from a bag, one after the other, and the first is not replaced.
State whether the two events are independent or dependent. Justify your answer.
State whether the two events are independent or dependent. Justify your answer.
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Answer
They are dependent.
Removing the first counter changes how many counters are left, so it changes the probability for the second.
Removing the first counter changes how many counters are left, so it changes the probability for the second.
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6
A fair coin is flipped and a fair die is rolled.
State whether 'the coin lands on heads' and 'the die shows a ' are independent. Justify your answer.
State whether 'the coin lands on heads' and 'the die shows a ' are independent. Justify your answer.
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Answer
They are independent.
The result of the coin flip has no effect on the result of the die roll.
The result of the coin flip has no effect on the result of the die roll.
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7
A student flips a fair coin and it lands on heads times in a row.
The student says the next flip is more likely to be tails because tails is overdue.
Name the fallacy the student is using and explain why the student is wrong.
The student says the next flip is more likely to be tails because tails is overdue.
Name the fallacy the student is using and explain why the student is wrong.
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Answer
The gambler's fallacy (also called the Monte Carlo fallacy).
The coin has no memory of previous flips. Each flip is independent, so the probability of tails on the next flip is still .
The coin has no memory of previous flips. Each flip is independent, so the probability of tails on the next flip is still .
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8
A spinner has equal sections numbered to .
Calculate the probability of spinning a number less than or a number greater than .
Calculate the probability of spinning a number less than or a number greater than .
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Answer
Less than : — that is sections.
Greater than : — that is sections.
These cannot both happen, so
Greater than : — that is sections.
These cannot both happen, so
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