Answer:
There are six possible forms of 4-digit numbers consisting of two odd digits and two even digits:
OOEE
OEOE
OEEO
EEOO
EOEO
EOOE
Each of the three forms starting with (O)dd digits can form 5^4 = 625 numbers, since there are 5 odd and 5 even digits.
Each of the three forms starting with (E)ven digits can form 4*5^3 = 500 numbers, since the first digit cannot be zero.
Therefore, the total numbers possible is 3*(625+500)
= 3,375
Explanation: