Multiply by 9, 99 or 999

From the sutra Ekanyūnena Pūrveṇa, By one less than the one before, in Bharati Krishna Tirtha's 1965 book Vedic Mathematics. Deep dive into the sutra

Try it with your numbers

Number Nines (9, 99, 999)

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When to use it

Multiplying by a string of nines that has at least as many digits as the other number: 42 × 99, 357 × 999.

The idea

Left part: one less than the number. Right part: subtract that left part from the nines (all from 9).

Worked example: 42 × 99

1. 41 | One less than 42 is 41. That is the left part.
2. 41 | 58Right part: 99 − 41 = 58 (all from 9).
3. 4158So 42 × 99 = 4158.

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.