Answer: he can make 21 different numbers.
Step-by-step explanation:
If he uses only one card, the numbers are:
1, 2 and 6, so here we have 3 combinations.
If he uses two cards, for the first digit he has 3 options (because he has 3 cards) and for the second digit he has two options (because he already selected one of the cards) then he has:
3*2 = 6 different options
If uses the 3 cards, then he has 3 options for the first card, 2 options for the second card and one option for the third card, then the number of combinations is:
c = 3*2*1 = 6 combinations.
Adding all together we have C = 3 + 6 + 6 = 21 combinations.
