Try the different numbers - there are only 11 after all - and multiply their digits. Or analyze the factors of the number 21.
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.
67/72 of them.
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 .
from 3 digits (10x10) to 4 digits (99X99)
89,999 different numbers i guess
You had me until "product." The product of 4 digits can't be prime.
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.
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.
In the US and Canada, telephone numbers have 10 digits, including the area code. Other countries use phone numbers with different numbers of digits.