Multiplying a number by 15

General rule:

Let us assume the number to be ABC, then thumb rule says,

Step 1: Keep on adding the half of left most digit and write the number on the right of the digit till all numbers are covered.

Note: Ignore fractional values.

For instance, A + (A/2) = X (say)

This forms the first digits of the answer.

Step 2: Next digit (B): B + (B/2) = Y

Step 2.a: If the number to left of B is odd, then 5 to the resultant (Y)

This forms the next digits of our answer.

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Similarly, by taking next digit (C), we have

(C) + (C/2) = Z

Follow the step 2.a as the case may be.

This forms the last part of our answer.

Note: If the resultant is two digit number from step 2 onwards, add the tens digit of resultant to the immediate left number.

Step 3: If the unit digit is

Even: then final answer ends with 0

Odd: then final answer ends with 5

For better understanding, let us take an example:

Example 1: 112 × 15

1 + (1/2) = 1

1 + (1/2) = 1 + 5 = 6 ( left digit is 1, so added 5)

2 + (2/2) = 3 + 5 = 8 ( left digit is 1, so added 5).

Following step 3, we have the unit digit of given number as 2. The final answer will end with 0.

Therefore, the required final answer is 1680.

Example 2: 211 × 15

2 + (2/2) = 3

1 + (1/2) = 1

1 + (1/2) = 1 + 5 = 6

Therefore, the required final answer is 3165

Lastly, let me tell you: it is nothing but simple algebra.

ABC × 15 = (ABC{0/5}) + (ABC/2)

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