Multiplying by 11

$64\times 11=6(6+4)4=704$For two-digit numbers, add digits and insert between.
Example:

64×11=6(6+4)4=704

Note: Works directly only for 2-digit numbers. For 3+ digits, the trick extends but requires carrying.

$523\times 11=5(5+2)(2+3)3=5753$

But notice $2+3=5$ was fine — if it were ≥ 10, you’d need to carry.
Example: 

$Step\; 1\; –\; Write\; the\; first\; digit\; (7).$

$Result:$

Step 1 - Write the first digit (7).

Result: 7

Step 2 – Add adjacent digits:

$7+8=15$ → write 5, carry 1

So we have:$7\ \left(15\right)\ \left(12\right)\ 4$
7 (15) (12) 4 → after carrying: