Praxis Core Academic Skills for Educators: Mathematics (5733)Number and QuantityMedium
A chemist is preparing a solution by mixing two chemicals in a ratio of 3:5. If the total volume of the solution is 240 mL, how much of the second chemical is needed?
- A144 mL
- B90 mL
- C120 mL
- D150 mL
Show answer & explanationAnswer & explanation
Correct answer: D. 150 mL
The ratio 3:5 means there are 3 parts of the first chemical and 5 parts of the second, for a total of 3 + 5 = 8 parts. To find the volume of one part, divide the total solution volume by the total parts: 240 mL / 8 parts = 30 mL/part. Since the second chemical is 5 parts, multiply 5 by 30 mL/part = 150 mL.
Why the other options are wrong
- A. This might come from a miscalculation, perhaps using 3/5 of 240mL, which is incorrect.
- B. This is the amount of the first chemical (3 parts * 30 mL/part).
- C. This might come from an incorrect calculation of parts or an arithmetic error.
Ratio and Proportion
A ratio compares two quantities, while a proportion states that two ratios are equal. Ratios can be used to divide a total quantity into parts.
- Ratios can be written as a:b, a/b, or 'a to b'.
- The sum of the parts in a ratio represents the total number of 'shares' or 'units'.
- To find the value of one part, divide the total quantity by the sum of the ratio parts.
Memory trick: Parts add up, then multiply for the final cup!