Let A equal the number of four digit odd numbers. Let B equal the number of four digit multiples of 3. Find A + B.

There are 9000 four-digit natural numbers, of which (9000 : 2 =) 4500 are odd.

A = 4500

There are (9000 : 3 =) 3000 four-digit natural numbers that are multiples of 3.

B = 3000

Hence 4500 + 3000 = 7500.