532
Try the different numbers - there are only 11 after all - and multiply their digits. Or analyze the factors of the number 21.
two
If you mean, "What is the largest number of digits possible in the product of two 2-digit numbers" then 99 * 99 = 9801, or 4 digits. Anything down to 59 * 17 = 1003 will have 4 digits.
In the everyday decimal system of writing numbers . . .-- The largest two digits are 8 and 9.-- Their product is 72.-- 10 less than that is 62 .
67/72 of them.
Forming three three digit numbers that use the numbers 1-9 without repeating, the highest product possible is 611,721,516. This is formed from the numbers 941, 852, and 763.
from 3 digits (10x10) to 4 digits (99X99)
You had me until "product." The product of 4 digits can't be prime.
It's not entirely clear from the wording of your question what it is you want to know. Seven is a one-digit number. The sum of its digits is 7. Seven has two factors: 1 and 7. That sum is 8.
89,999 different numbers i guess
Zero.
There are 7,290 different 4-digit numbers that can be formed from the digits 1-9 without repetition.