Có 20 tấm thẻ được đánh số từ 1 đến 20. Lấy ngẫu nhiên hai thẻ từ 20 tấm thẻ đó. Xác suất để tổng hai số thẻ ghi trên hai tấm thẻ đó là một số lẻ bằng?

To solve this problem, we can use the concept of probability. The total number of ways to choose 2 cards out of 20 is given by the combination formula: $$ where n is the total number of items and k is the number of items to choose. The total number of ways to choose 2 cards out of 20 is: $$ Now, let's find the number of ways to choose two numbers such that their sum is odd. If one card has an even number, then the other card must have an odd number for their sum to be odd. There are 10 even numbers (from 1 to 20), so there are 10 choices for the first card. After choosing an even number for the first card, there are 10 odd numbers left as choices for the second card. So, the total number of ways to choose two numbers such that their sum is odd is: $$ Therefore, the probability that the sum of two randomly chosen numbers from 1 to 20 is an odd number is: $$ So, the final answer matches with our calculated probability.