f5數學題兩條? 10 點

A 3-digit number is formed from digits 0,2,3,4,5,7,9.Each digit can be used only once. (total number of ways = 7P3 = 7*6*5 = 210)

1. last digit of even numbers must be 0,2,4
total number of even numbers = 6*5*3 = 90

2. first digit of odd numbers that are greater than 400 must be 4,5,7,9
last digit must be 3,5,7,9

We have to seperate into 2 cases
Case1: first digit is 4
1 choice for the first digit, 5 for the middle one, 4 for the last one
number of odd numbers = 1*5*4 = 20

Case2: first digit is 5,7,9
3 choice for the first digit, 5 for the middle one, 3 for the last one
number of odd numbers = 3*5*3 = 45

total number of odd numbers that are greater than 400 = 20+45 = 65