Let A equal the number of four digit odd numbers. Let B equal the number of four digit multiples of 3. Find A + B.
How many 4-digit numbers are there? Well, you have:9,999 - 1,000 + 1 =9,000 such numbers. How many of them are odd? Half of them, or: 9,000 / 2 =4,500 odd integers. How many of them are multiples of 3? 9,000 / 3 =3,000. So A + B =4,500 + 3,000 =7,500. And that is it.