Examine three consecutive weeks (21 days). In this 21-day-long period the lumberjack cut off exactly 36 trees. Considering the fact that he cut off at least one tree every day and we examine how many trees he has cut till the kth day we can say that we have 21 different numbers (between 1 and 36). Among these numbers there are two numbers that are congruent modulo 20 (using pigeonhole principle). Between these two days he cut off exactly 20 trees.
I hope this prove is clear enough and I could help you.