Final answer:
The correct temperature at an altitude of 21,000 feet, given a decrease of 3 degrees per 1,000 feet, would be -12 degrees. However, this is not an available option, indicating a possible typo in the question's options. Clarification is needed to provide an accurate answer.
Step-by-step explanation:
To find the temperature at an altitude of 21,000 feet when the temperature drops by 3 degrees for every thousand feet, we can use a straightforward mathematical calculation. For every 1,000 feet the plane ascends, the temperature drops by 3 degrees. Multiplying the rate of temperature drop by the number of 1,000 feet increments in 21,000 feet gives us:
Determine the number of 1,000-foot increments in 21,000 feet: 21,000/1,000 = 21 increments.
Calculate the total drop in temperature: 21 increments × 3 degrees/increment = 63 degrees.
Subtract this value from the ground temperature: 51 degrees - 63 degrees = -12 degrees.
The calculated temperature would, therefore, be -12 degrees, which is not an option in the question. However, there seems to be a typo in the question. If we were to calculate based on the options provided, the correct answer would be 51 degrees - (3 degrees × 21) = 51 degrees - 63 degrees = -12 degrees. Since -12 degrees is not an option and the question possibly has a typo, we need to clarify the actual temperature drop rate or the correct altitude to provide a precise answer.