Problem 28 - Applied Mathematics: For the Managerial, Life, and Social Sciences

Question

Classify each problem according to whether it involves a permutation or a combination. A member of a book club wishes to purchase two books from a selection of eight books recommended for a certain month. In how many ways can she choose them?

Step-by-Step Solution

Answer

This problem involves a combination, since the order in which the books are chosen does not matter. The member of the book club can choose two books from the selection of eight books in 28 different ways, calculated using the combination formula: \[ C(8, 2) = \binom{8}{2} = \frac{8!}{2!(8-2)!} = 28 \]

Step 1: Understand permutations and combinations

Permutations and combinations are both ways to count different arrangements of objects. The primary difference between them is that permutations are concerned with the order of the objects, while combinations are not. In other words, if the order matters, it is a permutation, and if the order does not matter, it is a combination. Step 2: Classify the given problem

Step 2: Determine if it is a permutation or a combination

In this situation, a member of a book club wants to purchase two books from a selection of eight books. The order in which they choose the books does not matter (i.e., choosing Book A then Book B is the same as choosing Book B then Book A), and each book can be chosen only once. Therefore, this problem involves a combination. Step 3: Calculate the number of ways based on the classification

Step 3: Calculate the number of ways to choose two books from eight

Since we have concluded that this problem involves a combination, we can use the formula for combinations to calculate the number of ways the books can be chosen: \[ C(n, k) = \binom{n}{k} = \frac{n!}{k!(n-k)!} \] where C(n, k) represents the number of combinations of n items taken k at a time. In our case, n = 8 (number of books to choose from) and k = 2 (number of books to be chosen). Plugging these values into the formula, we get: \[ C(8, 2) = \binom{8}{2} = \frac{8!}{2!(8-2)!} = \frac{8!}{2!6!} = \frac{8\times 7}{2} = 28 \] So, the member of the book club can choose two books from the selection of eight books in 28 different ways.

Key Concepts

Mathematical Counting

Understanding mathematical counting is fundamental for solving a wide array of problems in mathematics and related fields. It's all about determining the number of ways in which a set of items can be arranged, selected, or combined.

In a typical exercise, such as deciding how many ways a member of a book club can select two books from a provided list, it’s essential to determine whether the scenario requires permutations or combinations. This distinction arises from whether we consider the order of selection to be important. Permutations would apply if the order mattered, but in our book club example, we use combinations because the two books, regardless of the order in which they are chosen, result in the same outcome for the reader.

Mathematical counting methods are indispensable tools for making these determinations and ensuring that problems are correctly classified and solved.

Factorial Notation

Integral to the concept of permutations and combinations is factorial notation, expressed as '!' following a number. A factorial represents the product of all positive integers up to that number. For instance, '5!' (read as 'five factorial') would be calculated as \(5 \times 4 \times 3 \times 2 \times 1 = 120\).

When calculating the number of combinations or permutations, factorials often come into play for determining the total number of ways to arrange or select items without repetitions.

It's important to note that by definition, '0!' is equal to 1 — this might seem counterintuitive, but plays a crucial role in arranging formulas correctly and ensuring accuracy in calculations involving mathematical counting.

Binomial Coefficient

The binomial coefficient is a pivotal concept used in counting the number of combinations of items. It is represented using the notation \(\binom{n}{k}\), where 'n' is the total number of items to choose from, and 'k' is the specific number of items being chosen.

This coefficient is calculated using the formula \(\binom{n}{k} = \frac{n!}{k!(n-k)!}\), effectively telling us how many ways 'k' items can be chosen from a set of 'n' items without regard for order.

Consistent with our book club problem, the binomial coefficient allows us to effectively ignore the order in which books are chosen. Calculating the binomial coefficient for 2 books out of 8, we ascertain that there are 28 unique combinations — providing a clear, quantitative understanding of the problem at hand.