Math Problem Statement

An ordinary die is rolled twice, and the sum is recorded. Compute the probability of: (a) The sum is greater than 7, (b) The sum is not divisible by 5 and not divisible by 6.

Solution

The question involves calculating probabilities for specific events concerning the roll of two six-sided dice. Here are the steps to compute the probabilities:

Event A: The sum is greater than 7.

  1. Total possible outcomes when two dice are rolled = 6 * 6 = 36.
  2. Favorable outcomes for sum > 7 (i.e., sums of 8, 9, 10, 11, 12):
    • Sum = 8: (2,6), (3,5), (4,4), (5,3), (6,2) → 5 outcomes.
    • Sum = 9: (3,6), (4,5), (5,4), (6,3) → 4 outcomes.
    • Sum = 10: (4,6), (5,5), (6,4) → 3 outcomes.
    • Sum = 11: (5,6), (6,5) → 2 outcomes.
    • Sum = 12: (6,6) → 1 outcome.
    • Total favorable outcomes = 5 + 4 + 3 + 2 + 1 = 15.

Event B: The sum is not divisible by 5 and not divisible by 6.

  1. We already know total possible outcomes = 36.
  2. Sums from two dice that are divisible by 5 or 6:
    • Divisible by 5: 5, 10, 15, 20 → (5 is not possible, 15 and 20 are not possible with two dice, only 10 is valid: (4,6), (5,5), (6,4) → 3 outcomes).
    • Divisible by 6: 6, 12, 18 → (6: (1,5), (2,4), (3,3), (4,2), (5,1), 12: (6,6) → 7 outcomes).
    • Total for divisible by 5 or 6 = 3 + 7 = 10 outcomes.
    • Overlapping outcomes are only (6,4) and (4,6) from sum = 10, counted once in the divisible by 5 set.
    • Corrected count = 10 - 1 = 9.
    • Outcomes not divisible by either 5 or 6 = 36 - 9 = 27.

Now we can calculate the probabilities:

  • Probability of Event A (P(A)) = Favorable outcomes for A / Total possible outcomes = 15 / 36.
  • Probability of Event B (P(B)) = Outcomes not divisible by either 5 or 6 / Total possible outcomes = 27 / 36.

Let's compute these probabilities and round to two decimal places as requested.The probabilities for the events given are:

  • P(A)P(A) (The sum is greater than 7): 0.42
  • P(B)P(B) (The sum is not divisible by 5 and not divisible by 6): 0.75

Would you like to delve into any details or have any other questions on this?

Here are some related questions you might find interesting:

  1. How would the probabilities change if the dice were not fair?
  2. What is the probability of rolling a sum less than 7?
  3. Can you calculate the probability of rolling an even number sum?
  4. What are the chances of rolling a sum divisible by 3?
  5. How do these probabilities translate to practical scenarios like games involving dice?

Tip:

Remember, in probability, knowing the total number of outcomes and how to categorize events based on those outcomes is crucial to determining the likelihood of any event.

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Math Problem Analysis

Mathematical Concepts

Probability
Combinatorics

Formulas

Probability of an event = (Favorable outcomes) / (Total outcomes)
Total outcomes for two dice = 6 * 6 = 36

Theorems

Basic Probability Theorem

Suitable Grade Level

Grades 9-12