### Instruction: What is the largest four-digit number whose digits add up to 16?### Response: I want to find the largest four-digit number, so I should start by looking at the most significant digit, the one in the thousands place.
To make it as large as possible, I should pick the largest digit that does not exceed 16, which is 9.
That means the other three digits must add up to 16 - 9 = 7.
The next most significant digit is the one in the hundreds place.
To make it as large as possible, I should pick the largest digit that does not exceed 7, which is 7.
That means the other two digits must add up to 7 - 7 = 0.
The next most significant digit is the one in the tens place.
To make it as large as possible, I should pick the largest digit that does not exceed 0, which is 0.
That means the last digit must also be 0.