ISEE Word Problems Question 245: Answer and Explanation
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Question: 245
9. Perry and Esther are driving towards each other, each at a constant rate. Perry began his drive two hours before Esther began hers. Esther is driving at a slower speed than Perry. Which additional piece of information, if given, would allow a determination of how long Perry had been driving before he and Esther met?
- A. the difference between Perry and Esther's speeds
- B. the sum of Perry and Esther's speeds
- C. Perry's speed
- D. Esther's speed
Correct Answer: B
Explanation:
The answer is B
When two objects are moving in the same direction, you can subtract their rates to determine the rate of one object relative to the other. When two objects are moving in the opposite direction, as they are here, you can add the rates to determine the rate of one object relative to the other. As rate × time = distance, knowing the sum of their rates and the distance between them will allow you to calculate the time it will take for them to meet.