ABCD is a number.
The number will odd, only if it last digit is odd - , so for D, we have 2 choices {1 and 3}.
For A, now, we could have 4 choices (because each digit should be different), but we cannot use "0", so it is going to be only 3 choices.
For B, we can use "0", and we already used 2 digits, so it is going to be 3 choices.
For C, we used already 3 digits, so we have left only 2 choices.
So altogether,
2*3*3*2=36 (4 digit odd numbers).
