Joana wrote the numbers from 1 to 10000 on the blackboard and then erased all multiples of 7 and 11. What number remained in the 2008th position?
Problem 803
Official solution
Initially observe that, from 1 to 77, Joana erased 11 multiples of 7 and 7 multiples of 11. Since 77 is a multiple of 7 and 11, she erased numbers, leaving numbers. Now, grouping the first 10000 numbers into groups of 77 consecutive numbers, this reasoning applies to each of the lines below, that is, in each line 60 numbers remained.
Since , we know that among the first numbers, numbers remained without being erased.
We still need to count 28 numbers. Let's then examine the 34th line, which starts with 2542. Since the erased numbers are in columns , etc., and up to the column, eight numbers have been erased, there remain numbers in the line. Therefore, after erasing the multiples of 7 and 11 in this line, the 28th number is 2577. Thus, the number in the 2008th position is 2577.