1. **State the problem:** Find a number greater than 40 and less than 52 that has 3 ones in it, where "3 ones" means 3 units (ones) in the number's value, not the digit '1'. 2. **Analyze the problem:** The number must be between 40 and 52. 3. **Interpret the phrase "3 ones":** This means the number has 3 ones in its place value, i.e., the units digit is 3. 4. **Check numbers between 40 and 52 with units digit 3:** These are 43 only. 5. **Conclusion:** The number is 43 because it has 4 tens and 3 ones (units). **Final answer:** The number Andrew is thinking of is $43$.