Answer
30
Work Step by Step
In the sequence of six digits,
there will be one "1", one "2" and the other four digits are to be "0"
We apply a strategy similar to that of example 3.
(We select slots where to place digits)
STEP 1 pick a slot (of six) to place a "1".... 6 outcomes
STEP 2 pick a slot (of the five remaining) to place a "2".... 5 outcomes
STEP 3 pick 4 slots to place the zeros (1 way to to do this)... 1 outcome
Multiplication principle: TOTAL = 6(5)(1)=30 outcomes
