Answer:
20
Step-by-step explanation:
You have to fill in 6 slots _ _ _ _ _ _.
Since first digit must be 4, there is only 1 way to fill in the 1st slot.
with the 5 slots, we fill in the 6s first, 3 of them.
So we have 5 options and 3 items to fill, (5 choose 3).
Then 2 ways to fill 1 and 1 way to fill 5. (It doesn't matter if you pick 5 or 1 first).
So it becomes (5C3) * (2C1) *(1C1)
= 1 * 5!/(3!)(5-3)! * (2) * 1
= 20
