Five cards are written with numbers 1, 2, 3, 4 and 5 respectively. If two of the five cards are randomly selected, the probability that the sum of the numbers on the two cards is odd is 0______ ．

Five cards are written with numbers 1, 2, 3, 4 and 5 respectively. If two of the five cards are randomly selected, the probability that the sum of the numbers on the two cards is odd is 0______ ．

According to the meaning of the question, this probability model is a classical probability model. Two cards are randomly selected from five cards. There are 10 common methods, C52 = 10, and the odd sum of the numbers on the two cards is c31c21 = 6. Therefore, according to the calculation formula of classical probability, the probability that the odd sum of the numbers on the two cards is 610 = 35

Xiao Li wrote a positive number on each of the four same cards, randomly selected two of them, added the numbers on them, and repeated this, each time the sum was equal

Analysis: all four cards are positive numbers, and the number of each card is less than 5
And two of them have a sum of 8, so there are at least two cards with 4
7 = 4 + 3, at least one card says 3,
With 3, the number 1 cannot appear
The same: can't have two card numbers less than 3, there must be only one 2
So: the four positive numbers written on the four cards are: 2, 3, 4, 4

Five cards are written with numbers 1, 2, 3, 4 and 5 respectively. If two of the five cards are randomly selected, the probability that the sum of the numbers on the two cards is odd is 0______ ．

According to the meaning of the question, this probability model is a classical probability model. Two cards are randomly selected from five cards. There are 10 common methods, C52 = 10, and the odd sum of the numbers on the two cards is c31c21 = 6. Therefore, according to the calculation formula of classical probability, the probability that the odd sum of the numbers on the two cards is 610 = 35