dandanapple

1st choice: 10 possibilities
2nd choice: 8 possibilities (removing one family)
3rd choice: 6 possibilities (removing the second possibilites)

Then among the chosen 3, no need to differentiate the 1st, 2nd and 3rd: final answer = (10*8*6)/3! = 80

dandanapple

C53=5*4*3.

Then you need to devide the product by 3! to remove the overcounting.

dandanapple

(10*8*6)看懂了，为什么要除以3！呢？
不好意思，我的排列组合很混乱。