Indian Multiplication (Tens, 11 to 19)

Multiply two numbers from 11 to 19 using the Indian mental-math trick. See each step: add one number to the other's ones digit, times ten, plus the product of the ones digits.

Input

Multiply two numbers from 11 to 19 using the Indian mental-math method.

First number

Second number

Enter whole numbers from 11 to 19 for both.

Result

13 x 14 equals

182

(13 + 4) x 10 + 3 x 4 = 170 + 12 = 182

Intermediate values

Add 4 to 13	17
Multiply 17 by ten	170
Product of the ones digits 3 x 4	12

Mental-math steps

Add the second number's ones digit 4 to the first number 13, giving 17.

Multiply 17 by ten to get 170.

Multiply the ones digits 3 x 4 to get 12.

Add 170 and 12 to get the answer 182.

How it works

For two numbers from 11 to 19, writing a=10+p and b=10+q gives a*b=(a+q)*10+p*q.
The steps are: add the other number's ones digit to one number, multiply by ten, multiply the two ones digits, then add the two results.
Example: for 13*14, (13+4)*10=170 and 3*4=12, so the answer is 170+12=182.
Inputs must be whole numbers from 11 to 19. Values outside this range are not calculated.
This trick is the expansion (10+p)(10+q)=100+10p+10q+pq, reordered to be easy to do in your head.

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