There are five sheets of cards of 0, 1, 1, 2, 3.
I take out two sheets out of five and make two digits of integers.
How many different integers are there in all?
Answer
10
Solution
When I use two pieces of 1, the integer to be able to make is 11.
Consider about cases to use 1 one time or zero.
As for tens digit, there are three pieces of numbers of 1, 2 or 3.
As for ones digit, there are three pieces of numbers which are not used in tens digit and 0.
Thus, there are 3 × 3 = 9 ways.
Accordingly there are 1 + 9 = 10 in all.
