3.25: Odd 5-digit numbers greater than 70 000

A 5-digit number is made using the digits 0, 1, 2, 3, 4, 5, 6, 7, 8 and 9. No digit may be used more than once. Find how many such 5-digit numbers are odd and greater than 70 000.

The number of combinations of $n$ objects taken 3 at a time is 2 times the number of combinations of $n$ objects taken 2 at a time. Find the value of $n$.