The correct option is B 671
Least digit used = 4
∴ We can use 4,5,6,7,8,9. But remember that at least one 4 must be used.
------(4 blanks)
1st blank can be filled in 6 ways.
2nd blank can be filled in 6 ways.
3rd blank can be filled in 6 ways.
4th blank can be filled in 6 ways.
4 blanks can be filled in 64 ways. But out of these, some may contain no 4 at all. Let us find them.
-------(4 blanks)
Each blank can be filled in 5 ways (by 5, 6, 7, 8, or 9)
∴54 ways (no 4 at all)
∴ Ans =64−54 (at least one 4)
=671