Praxis Core Academic Skills for Educators: Mathematics (5733)Number and QuantityMedium
A scientist is observing a chemical reaction where the temperature decreases by 2.5°C every 15 minutes. If the initial temperature is 20°C, what will the temperature be after 1 hour and 15 minutes?
- A5°C
- B10°C
- C7.5°C
- D2.5°C
Show answer & explanationAnswer & explanation
Correct answer: C. 7.5°C
First, calculate the total time in 15-minute intervals. 1 hour and 15 minutes is 75 minutes. Divide 75 by 15 to get 5 intervals. Then, multiply the temperature decrease per interval (2.5°C) by the number of intervals (5) to find the total decrease. Finally, subtract the total decrease from the initial temperature.
Why the other options are wrong
- A. This option results from an incorrect calculation of the total decrease, possibly by using 6 intervals or miscalculating the decrease per interval.
- B. This option results from incorrectly calculating the total decrease, possibly by using 4 intervals (1 hour) instead of 5.
- D. This option would be correct if the temperature decreased by 2.5°C per hour, which is not the case.
Repeated Subtraction in Intervals
This concept involves calculating a total change over time by repeatedly subtracting a fixed amount for each discrete time interval.
- Determine the total elapsed time.
- Identify the duration of each interval.
- Calculate the number of intervals that occur within the total time.
- Multiply the change per interval by the number of intervals.
- Apply the total change to the initial value.
Memory trick: Track the time, then decrease, to see the final temperature's ease.