The 11 Trick is one of the easiest tricks out there, but it can be INCREDIBLY useful on the number sense test. When multiplying any number greater than 10 by 11, there's a very easy process to do that gets you the right answer with practically no effort. For example:
65 x 11
The number in the units place of the answer will be 5. For the middle number, add 6 and 5 together, to get 11. Place a 1 in the tens place of the answer and carry the one over to the 6, which becomes a 7, for the hundreds place.
To make it more simple, when multiplying 11 by the number A'B, you will end up with answer A'A+B'B. This works with longer numbers as well - simply continue adding the middle two numbers, starting from the right, and carry over the remainders. Here are some other examples:
43 x 11 = 473
89 x 11 = 979
129 x 11 = 1419
462 x 11 = 5082
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