Question

how would we multiply the numbers 13680347 with 9 mentally

Answer 1

multiply the no. by 10 we get 136803470 or simply add 0 in ones place.

Answer 2

To multiply 13680347 by 9

First multiply it by ten by simply adding a zero next to the number
= 13680347

Now subtract the number 13680347 from 136803470
= 123123123.
-----------------–----–-------------------
Hence your answer is 123123123