Answer:
b. 504
Step-by-step explanation:
In this case, we have to apply the permutation formula, since the order is important and it cannot be repeated. The formula is as follows:
nPr = n! / (n-r)!
From 0 to 9, there are 10 digits, but we must remember that the number 7 has already been used and cannot be repeated, therefore n = 9. And they are 4 digits but only 3 are needed, therefore r = 3.
Replacing:
9P3 = 9! / (9-3)!
nPr = 504
Therefore the answer is b.
