Write the greatest 4-digit number and express it in terms of its prime factors.

The greatest 4-digit number = 9999
Now, 9999 = 3 x 3333
Also, 3 = (1 x 3) and 3333 = (3 x 1111)
1111 can be written as (101 x 11)
Hence, we get the factor tree as below:



Hence, the prime factorization of 9999 is $3\times 3\times 11\times 101.$