14
221
11 and 18
11 and 16.
11 is a tricky one, but this is how:if the sum of every other digit minus the sum of all the other digits equals a multiple of 11example:Q: Is 164725 divisible by 11?A: Yes. and this is why... first you take every other digit 164725, then add them 6+7+5=18. Now take the rest of the numbers 164725 and add them, 1+4+2=7. After that subtract them, 18-7=11, and 11 is a multiple of 11 (11x1=11).
21
You can select 12 numbers for the first digit, 11 numbers for the second digit, and 10 numbers for the third digit; so 12*11*10 = 1320 sets of 3 numbers can be made out of 12 different numbers.
221
14
99990 Note that if you add 11 to my answer you get a 6-digit number therefore it must be the largest 5-digit whole multiple of 11.
11
11
Eleven digit numbers are those in the range between 9,999,999,999 and 100,000,000,000 exclusive. There are 90,000,000,000 of these.
There are more 12-digit palindromic numbers than 11-digit palindromic numbers. This is because the number of possible 12-digit palindromic numbers is greater than the number of possible 11-digit palindromic numbers. In general, the number of palindromic numbers of length n is 9 * 10^((n-1)/2), so for 11-digit palindromic numbers, there are 9 * 10^5 = 900,000 possibilities, while for 12-digit palindromic numbers, there are 9 * 10^6 = 9,000,000 possibilities.
11 and 18
11 and 13.
Hint, but not your answer: You know your answer has to be between 11 and 99. Furthermore, it has to be a square. One, two, and three when squared are still single digit numbers. Four squared is 16, which is not squaring the ones digit. Keep going...
All the two digit prime numbers that can be reversed are 11, 13 and 31, 17 and 71, 37 and 73, and 79 and 97.