In the 4-digit number X3Y7, (X and Y represents unknown digit)
1. What is the largest number?
2. What is the smallest number?
3. What number is closest to 3,590?
1. To make a largest number, replace both X and Y with the largest digit 9 , the result is 9,397;
2. Replace X with 1, which is the smallest digit possible for a 4-digit number; and replace Y with 0, the result is 1307;
3. Replace X with 3, and replace Y with 9. Thus, the number closest to 3590 is 3,397