Question: 1. The mean of five numbers is 24. If one of the numbers is removed, the mean of the remaining four numbers becomes 20. Which number was removed?
1. **Problem:** Find the number removed when the mean of five numbers is 24, and after removing one number, the mean of the remaining four numbers is 20.
2. **Formula:** Mean of $n$ numbers is given by $$\text{Mean} = \frac{\text{Sum of numbers}}{n}$$
3. **Step 1:** Calculate the sum of the original five numbers.
$$\text{Sum}_5 = 5 \times 24 = 120$$
4. **Step 2:** Calculate the sum of the remaining four numbers after removing one number.
$$\text{Sum}_4 = 4 \times 20 = 80$$
5. **Step 3:** Find the removed number by subtracting the sum of the remaining four numbers from the original sum.
$$\text{Removed number} = \text{Sum}_5 - \text{Sum}_4 = 120 - 80 = 40$$
6. **Answer:** The number removed is $40$.
**Final answer:** C. 40