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Applications of Algebra in Word Problems Study Guide (page 2)

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Updated on Apr 25, 2014

Mixture Problems

Mixture problems involve two or more different types of materials that will be combined to form one mixture. To solve mixture problems, use the clues in the problem to set up the let statements for the amounts of each type that will be mixed. Then, add the parts together and set the sum equal to the total mixture. Use the following example to help with these steps.

Example 3

Jane wants to mix $4 per pound coffee with $6 per pound coffee to get a mixture that costs $5.50 per pound. If the total mixture contains 10 pounds, how many pounds of the $4 per pound coffee should she buy?

Read and understand the question. Jane is mixing together two different types of coffee. Each type is a different price. You are looking for the number of pounds of the less expensive coffee as a final answer.

Make a plan. Write an expression for the amounts of each type of coffee using the information given. Then, multiply each by the price per pound. Set this equal to the total mixture, which is equal to $5.50 × 10 or 5.50(10).

Carry out the plan. Let x = the number of pounds of the $4 per pound coffee. Since there is a total of 10 pounds in all, then 10 – x = the number of pounds of the $6 per pound coffee. Next, write an equation that adds the cost of the two types of coffee and sets it equal to the total. The cost of the $4 coffee is 4x, the cost of the $6 coffee is 6(10 – x), and the total cost is 5.50(10). The equation is

  • 4x + 6(10 – x) = 5.50(10)

Simplify by using the distributive property:

  • 4x + 60 – 6x = 55

Combine like terms:

  • –2x + 60 = 55

Subtract 60 from each side of the equation:

  • –2x + 60 – 60 = 55 – 60

Simplify to get –2x = –5. Divide each side by –2.

  • x = 2.5

Because x represents the number of pounds of the $4 per pound coffee, Jane should buy 2.5 pounds.

Check your answer. To check this solution, first find the amount of the $6 per pound coffee. Using your solution, this should be 10 – 2.5 = 7.5 pounds. Next, multiply the number of pounds of each by the cost of each. $4 × 2.5 pounds is equal to $10 and $6 × 7.5 pounds is equal to $45. These amounts have a sum of $10 + $45 = $55, which was the cost of the total mixture. This answer is checking.

Coin Problems

Coin problems are similar to mixture problems in that you are taking different values of the various coins and mixing them together to get a total amount. Take the following example.

Sebastian has a total of $2.75 in his bank. He has only quarters and dimes, and has 10 more dimes than quarters. How many of each coin does he have in his bank?

Read and understand the question. This question is looking for the total number of quarters and the total number of dimes in a bank containing only quarters and dimes. There are 10 more dimes than quarters.

Make a plan.Write an expression for the number of quarters and the number of dimes in the bank. Multiply each of these expressions by the value of each type of coin. Then, add these expressions and set them equal to the total of $2.75.

Carry out the plan. Let x = the number of quarters and let x + 10 = the number of dimes. A quarter has a value of $0.25, so multiply 0.25 by x to get 0.25x. A dime has a value of $0.10 so multiply 0.10 by x + 10 to get 0.10(x + 10). Next, write an equation by adding these expressions and setting them equal to $2.75. The equation is 0.25x + 0.10(x + 10) = 2.75. Use the distributive property to eliminate the parentheses. The equation becomes 0.25x + 0.10x + 1 = 2.75. Combine like terms to get 0.35x + 1 = 2.75. Subtract 1 from each side to get 0.35x = 1.75. Divide each side by 0.35.

    • x = 5

Therefore, there are 5 quarters and 5 + 10 = 15 dimes.

Check your answer. Check this solution by making sure that the coins are equal to a total of $2.75. Five quarters has a value of $0.25 × 5 = $1.25, and 15 dimes has a value of $0.10 × 15 = $1.50. The sum is $1.25 + $1.50 = $2.75, so this answer is checking.

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