positive integers

The first digit must be 7, 6 or 5
If the first digit is 7,then other digits are formed by3 4 4 5 5 6,
there are 6!/ (2!*2!)=180 cases,
If the first digit is 6,then other digits are formed by 3 4 4 5 5 7,
there are 6!/(2!*2!)=180 cases,
If the first digit is 5,then other digits are formed by 3 4 4 5 6 7,
there are 6!/2!=360 cases,
So there are total 180+180+360=720 positive integers are suitable.

"How many positive integers n can we form using the digits 3,4,4,5,5,6,7if we want n to exceed 5,000,000?"
-> exceed .? that would turn to infinite架喎...