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https://gmatclub.com/forum/10-25-560-is ... 26300.html
We are given the expression \( 10^{25} - 560 \) and asked to determine which of the following numbers (11, 8, 5, 4, or 3) **does not divide** the result.
Let’s evaluate the divisibility of \( 10^{25} - 560 \) by each of these numbers.
### **Step 1: Divisibility by 5**
Any number \( 10^n \) (where \( n \) is a positive integer) is divisible by 5 since it ends in zero. So, \( 10^{25} \) is divisible by 5, and 560 is also divisible by 5 because \( 560 \div 5 = 112 \).
Thus, \( 10^{25} - 560 \) is divisible by 5.
- **Not the answer.**
### **Step 2: Divisibility by 4**
We check divisibility by 4 using the last two digits of a number. The last two digits of \( 10^{25} \) are "00", and the last two digits of 560 are "60".
Thus, \( 10^{25} - 560 \) ends in 40, which is divisible by 4.
- **Not the answer.**
### **Step 3: Divisibility by 8**
We check divisibility by 8 using the last three digits of the number. The last three digits of \( 10^{25} \) are "000", and the last three digits of 560 are "560".
Now, \( 1000 - 560 = 440 \), and 440 is divisible by 8 because \( 440 \div 8 = 55 \).
Thus, \( 10^{25} - 560 \) is divisible by 8.
- **Not the answer.**
### **Step 4: Divisibility by 11**
To check divisibility by 11, we apply the alternating sum rule. For a number to be divisible by 11, the alternating sum of its digits must be divisible by 11.
The alternating sum of the digits of \( 10^{25} \) is \( 1 - 0 + 0 - 0 + \dots + 0 = 1 \), and the alternating sum of the digits of 560 is \( 5 - 6 + 0 = -1 \).
Thus, the alternating sum of \( 10^{25} - 560 \) is \( 1 - (-1) = 2 \), which is **not divisible by 11**.
Therefore, \( 10^{25} - 560 \) is **not divisible by 11**.
- **Answer: 11**
### **Step 5: Divisibility by 3**
To check divisibility by 3, we sum the digits of the number. If the sum of the digits is divisible by 3, the number is divisible by 3.
The sum of the digits of \( 10^{25} \) is 1, and the sum of the digits of 560 is \( 5 + 6 + 0 = 11 \). So, the sum of the digits of \( 10^{25} - 560 \) is \( 1 - 11 = -10 \), and the absolute value is 10, which is **not divisible by 3**.
However, there can be a calculation mix-up, so let’s verify step-by-step manually
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