To multiply a two-digit number by 11, add the two digits and place the result in the middle. Example: 23 × 11 = 253.
Multiply the number by 10 and subtract the original number. Example: 7 × 9 = (7 × 10) - 7 = 63.
Square by multiplying the first digit by the next, append 25.
Determine divisibility based on the last digits (even, divisible by 4, or divisible by 8).
Move the decimal left for 10% or two places left for 1%. Example: 10% of 350 is 35.
Divide by 4 for 1/4, by 2 for 1/2, and then multiply by 3 for 3/4.
Multiply by 10 and divide by 2, 4, 8, respectively.