How many combinations are there if there are 3 numbers?

The formula is 9*9*9*9*

* * * * *

The above answer is wrong on two counts: First, there are ten digits to chose from, not nine. Second, the answer is in terms of permutations ie 1234 being different from 2341. However, both these are the same combination.

The correct answer is 10*9*8*7/(4*3*2*1) = 210.