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How do you calculate the probability of rolling an even number three times with a six-sided die?

To calculate the probability of rolling an even number three times, first find the probability of rolling an even number on a single roll, which is 1/21/2 for a fair six-sided die. Then, assuming the three rolls are mutually independent, multiply this single-roll probability by itself three times: 12×12×12=18\frac{1}{2} \times \frac{1}{2} \times \frac{1}{2} = \frac{1}{8}.

Conditions

  • The die is fair (six-sided, equally likely outcomes).
  • The three rolls are mutually independent.
  • The target event is rolling an even number on every one of the three rolls.

Reasoning, step by step

  1. Calculate the probability of a single even roll: P(even)=36=12P(\text{even}) = \frac{3}{6} = \frac{1}{2}.
  2. Identify the assumption of independence for the three rolls.
  3. Set up the multiplication rule for independent events: P(all three even)=P(even)×P(even)×P(even)P(\text{all three even}) = P(\text{even}) \times P(\text{even}) \times P(\text{even}).
  4. Substitute the value 12\frac{1}{2} into the equation.
  5. Perform the multiplication: 12×12×12=18\frac{1}{2} \times \frac{1}{2} \times \frac{1}{2} = \frac{1}{8}.
  6. Conclude that the probability is 18\frac{1}{8}.

Example

The video derives P(even roll on 6-sided die)=3/6=1/2P(\text{even roll on 6-sided die}) = 3/6 = 1/2. It then sets up P(rolling even 3 times)=1/2×1/2×1/2P(\text{rolling even 3 times}) = 1/2 \times 1/2 \times 1/2 and calculates the final result as 1/81/8.

Common misconceptions

  • Adding the probabilities instead of multiplying them (12+12+12=32\frac{1}{2} + \frac{1}{2} + \frac{1}{2} = \frac{3}{2}, which is impossible for a probability).
  • Forgetting to check the independence assumption before multiplying.
  • Calculating the probability of rolling an even number at least once instead of exactly three times.

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Answers are generated from source material and independently checked. Consult the original video or creator if something is unclear.