23. Footrace There are 72 runners in a 10-kilometer race. How many ways can the runners finish first, second, and third?
Table of contents
- 1. Intro to Stats and Collecting Data55m
- 2. Describing Data with Tables and Graphs1h 55m
- 3. Describing Data Numerically1h 45m
- 4. Probability2h 16m
- 5. Binomial Distribution & Discrete Random Variables2h 33m
- 6. Normal Distribution and Continuous Random Variables1h 38m
- 7. Sampling Distributions & Confidence Intervals: Mean1h 53m
- 8. Sampling Distributions & Confidence Intervals: Proportion1h 12m
- 9. Hypothesis Testing for One Sample2h 19m
- 10. Hypothesis Testing for Two Samples3h 22m
- 11. Correlation1h 6m
- 12. Regression1h 4m
- 13. Chi-Square Tests & Goodness of Fit1h 20m
- 14. ANOVA1h 0m
4. Probability
Counting
Problem 3.4.34
Textbook Question
34. Lottery Number Selection A lottery has 52 numbers. In how many different ways can six of the numbers be selected? (Assume the order of selection is not important.)

1
Step 1: Recognize that this is a combination problem because the order of selection does not matter. The formula for combinations is given by: , where n is the total number of items, r is the number of items to choose, and '!' denotes factorial.
Step 2: Identify the values of n and r from the problem. Here, n = 52 (total numbers in the lottery) and r = 6 (numbers to be selected).
Step 3: Substitute the values of n and r into the combination formula: .
Step 4: Simplify the factorials in the denominator. Compute (52 - 6) = 46, so the formula becomes: .
Step 5: Cancel out the common terms in the numerator and denominator. This simplifies to: . Compute this to find the total number of ways.

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Key Concepts
Here are the essential concepts you must grasp in order to answer the question correctly.
Combinations
Combinations refer to the selection of items from a larger set where the order does not matter. In this context, we are interested in choosing 6 numbers from a total of 52, which is a classic example of a combination problem. The formula for combinations is given by C(n, k) = n! / (k!(n-k)!), where n is the total number of items, k is the number of items to choose, and '!' denotes factorial.
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Factorial
A factorial, denoted as n!, is the product of all positive integers up to n. For example, 5! = 5 × 4 × 3 × 2 × 1 = 120. Factorials are essential in calculating combinations and permutations, as they help determine the total arrangements of a set of items. In the lottery question, factorials will be used to compute the number of ways to choose 6 numbers from 52.
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Binomial Coefficient
The binomial coefficient, often represented as C(n, k) or 'n choose k', quantifies the number of ways to choose k elements from a set of n elements without regard to the order of selection. It is calculated using the formula C(n, k) = n! / (k!(n-k)!). This concept is crucial for solving the lottery number selection problem, as it directly provides the number of combinations of 6 numbers from 52.
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