1
8
It is 9.
9999 is the largest 4-digit number.When added with 1 it becomes 10000. 9999 + 1 = 10000.
It is 9999 (if a one is added, it becomes a five digit number: 9999 + 1 = 10,000)
1
7
4312
The largest one digit number that has three factors is 9. Its factors are 1, 3, and 9.
To form the largest even number, the last digit must be even, which means it can be either 8, 4, or 2. Placing the largest even digit, 8, in the ones place, we can continue selecting the next largest digits in descending order until all digits are used.
4312
1