This problem was given to 9-graders. We write down numbers except for the ones which contain the digit c. Find the nth such number. I looked at the indications, but the algorithm used there stumped me. I will describe in here: if $c \ne 0$, then we transform the number n - 1 (since 0 is part of the series) in the 9th base. Then each digit in the representation of the 9th base which is bigger of equal than c and smaller than 9 will be incremented with 1. The number we obtain is the number we are looking for, BUT IN BASE 10. How? Could anyone explain it to me? Also, please, be patient since I have just started Competitive Programming and I am not really that good.