The number will always be a multiple of 11 because; if (for example) I have the number 13, and I switch the digits, I will then have the number 31. Then I would add the numbers in the tens column, as units and get the number 4. Then I would add the numbers in the units’ column giving me, the number 4 - again. The reason for this is that the two numbers in the tens and units columns are exactly the same. The first few numbers of the 11 times tables, have the same numbers in the tens and units column making it a member of the 11 times table.
If the two numbers add to make a number bigger than 100, they will still make a number in the 11 times tables because; if (for example) I have the number 89 when I switch the digits I will have the number 98. However, when I add the numbers I get the answer 187. 187 is made up of 17 units (17) and 17 tens (170). If I look at it like this –
17x10
+17x1
17x11
- I can see that 17 x 11 (189) will be in the 11 times tables.