24) I have at my disposal five digits equal to 1 and one digit equal to 2. Using all or some of these digits, how many different numbers can I construct?
(A) 15
(B) 21
(C) 24
(D) 26
(E) 27.
24) The answer is (D).

Indeed, using only the digit 1, I can construct the 5 numbers 1, 11, 111, 1111, 11111. Using the digit 2 as well, I can construct: one number with one digit (2), two numbers with two digits (by placing the digit 2 in the first or second position), three numbers with three digits, and so on, for a total of $5+1+2+3+4+5+6=26$ numbers.