The basic rule of combinatorics
Answers are for parents and teachers.
1
A school canteen has 3 first courses and 4 drinks. How many different sets?
12
2
How many three-digit numbers have distinct digits?
648
3
In how many ways can a leader and a deputy be chosen from 5 people?
20
4
In how many ways can one book be chosen from 2 Russian, 3 English and 5 history books?
10
5
Why does the first place have 9 choices and the second also 9 in numbers with distinct digits?
The first digit cannot be 0 (9 choices); the second is chosen from the 9 digits other than the first, including 0.