1. Let's consider 8xy set
2. y e [1,3,5,7,9] (5 possibilities), x has 10 possibilities - 2 possibilities (8 and y). The total number is 5*8=40
3. Let's consider 9xy set
4. y e [1,3,5,7] (4 possibilities), x has 10 possibilities - 2 possibilities (9 and y). The total number is 4*8=32
Great explanation walker! Thanks. The OA is C. I just have 1 question. Why did you have to start first from the left digit, then skipped the middle digit to go to the right digit, then back to the middle digit? Why couldn't you just go from left to middle, then from middle to right?