A store sells refrigerators 10, including 7 first-class products and 3 second-class products. Someone arrives at the store, and two refrigerators have been sold. Please ask this person to buy them.

It's like drawing lots. It doesn't matter which one. The third buyer of the refrigerator and the first buyer of the refrigerator have a probability of 7/ 10 to buy the first-class product.

The mathematical solution is as follows:

The first two people buy refrigerators in four situations: one, one, two, two, two. Find the sum of the probabilities of the third party buying the first-class products in four cases.

1 1: 7/ 10 * 6/9 * 5/8 = 2 10/720

One, two: 7/10 * 3/9 * 6/8 =126/720

2 1: 3/ 10 * 7/9 * 6/8 = 126/720

22: 3/ 10 * 2/9 * 7/8 = 42/720

Sum of the four: 210/720+126/720+126/720+42/720 = 504/720 = 7/10.