For multiplying any 4-digit number with another 4-digit number such that the sum of the initial digits of the multiplier and multiplicand = 1000 and the remaining digits of multiplier are same as that of the multiplicand.

For multiplying any 5-digit number with another 5-digit number such that the sum of the initial digits of the multiplier and multiplicand = 1000 and the remaining digits of multiplier are same as that of the multiplicand.

You cannot use this method to multiply numbers with unequal number of digits, i.e. multiplying a 4-digit number with a 5-digit number.

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Speed Math Shortcut - Multiplying Numbers with sum of Initial digits = 1000: Example 1

When multiplying 4-digit numbers with common last digit, remember to write the RHS of the answer in 2-digit form by prefixing a zero, if the value is single digit, e.g. write ‘9’ as ‘09’.

When multiplying 5-digit numbers with last 2 digits common, remember to write the RHS of the answer in 4-digit form by prefixing zero(s), if the value is less than 4-digit, e.g. write ‘9’ as ‘0009’ or ‘21’ as ‘0021’ or ‘121’ as ‘0121’.

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