Multiply by 11

From the sutra Ūrdhva Tiryagbhyām, Vertically and crosswise, with a multiplier of 11, in Bharati Krishna Tirtha's 1965 book Vedic Mathematics. Deep dive into the sutra

Try it with your numbers

Number

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When to use it

Any number times 11.

The idea

Keep the outside digits and fill the middle with the sums of neighbouring digits, carrying when a sum is 10 or more.

Worked example: 23 × 11

1. 2 _ 3Write the first digit (2) and the last digit (3) apart.
2. 2 5 3Neighbour sums go in between: 2+3=5.
3. 253No sums of 10 or more, so nothing to carry.

Practice this trick

All 15 speed tricks, each with its calculator. The speed tricks use sutras from Bharati Krishna Tirtha's 1965 book Vedic Mathematics; the lessons, problems and pictures here are our own.