The first two digits must be 6 and 4.
The third digit must be between 0 (inclusive) and 5 (exclusive), so there are 2 choices for it (0 or 2).
The remaining three digits have no additional restrictions. There are 3! = 6 ways to permute them.
Then there are 2•6 = 12 such numbers.
640 279
640 297
640 729
640 792
640 927
640 972
642 079
642 097
642 709
642 790
642 907
642 970
