dngboy89nh wrote:

Your number is 3***

You have 0 1 2 3 4 5 6 7 8 9; So eliminate 0 1 2 3; Remain S = {4, 5, 6, 7, 8, 9} (6 digits)

Because you need 3 distinct digits which increase from left to right, take 3 digits from S.

With 3 digits which increase from left to right : only one way to arrange; Ex: 468 (not 486)

It means: 6C3 = 6!/[3!x(6 - 3)!] = 6!/[3!x3!] = 4 x 5 x 6 / 3! = 4 x 5 = 20.

So the answer is A.

For those who could not understand why we using Combination Formula here:

We have to pick 3 different digits from the Set S = {4, 5, 6, 7, 8, 9}

We are using Combination because we are only selecting the digits as there is only

1 way to arrange them i.e. increasing order.

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