Ekanyunena Purvena: By one less than the previous one

Ekanyunena Purvena is the fourteenth sutra of Vedic mathematics. Its corollary is Dhvajanka. It shares some similarities with the first sutra, Ekadhikina Purvena, and the twelfth sutra, Shesanyankena Charamena, although the first and twelfth sutras hold far more practical applications than Ekanyunena Purvena.

This sutra is used when multiplying any number by a number consisting only of nines, such as 9, 99, 999, 9999, etc.

For the smaller numbers consisting of nines, it is usually easier to just round the number up to the nearest power of ten, add that many zeroes to the other number, than subtract the other number. ie. 9 x 5,000 = (10 x 5,000) - 5,000 = 45,000

For the larger numbers consisting of nines, this sutra might save time.
Example: 9999 x 1016
The left hand side of the answer will be found by subtracting 1 from 1016, which is 1015
The right hand side of the answer will be found by subtracting the bolded number in the previous step (1015) from the number consisting of nines: 9999 - 1015 = 8984
Thus, 9999 x 1016 = 10,158,984

Ekanyunena Purvena can also be used in some problems that call for converting certain fractions to decimals This will not arrive at every possible decimal point in every case.

Example: Fractions for which the denominator is 7
See Kevalaih Saptakam Gunyat.

Example: Fractions for which the denominator is 13
To solve this, we use Kalau Kshudasasaih: "For 13 the multiplicand is 077"
We would solve for 1/13 as follows:
We multiply the multiplicand (077) by 999. We use 999 because it has the same number of digits as our multiplicand and consists only of nines
077 x 999 = 076923
Multiply that number by the numerator: 1 x 076923 = 076923
Place a decimal point at the beginning: 0.076923
Thus, 1/13 = 0.076923

Example: Fractions for which the denominator is 17
To solve this, we use Kamse Kshaamadaaha-khalairmalaih: "For 17 the multiplicand is 05882353" (The literal translation is "In king Kamsa's reign famine and unhygenic conditions prevailed.")
We would solve for 1/17 as follows:
We multiply the multiplicand (05882353) by 99999999. We use 99999999 because it has the same number of digits as our multiplicand and consists only of nines
05882353 x 99999999 = 0588235294117647
Multiply that number by the numerator: 1 x 0588235294117647 = 0588235294117647
Place a decimal point at the beginning: 0.0588235294117647
Thus, 1/17 = 0.0588235294117647

RESOURCES:
Vedic Mathematics by Sri Bharati Krisna Tirthaji
http://www.vedamu.org/Mathematics/course.asp
http://www.sanalnair.org/articles/vedmath/intro.htm
http://www.vedicganita.org/ganitsutras.htm