Re: Helen and Sergey must cut down a row of trees. Helen can cut
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10 Dec 2017, 19:04
I dont know if my reasoning is right but I proceed as follows.
Let's pretend that the aforementioned row of trees is constitued of 21 trees.
Since Serguey needs 7 hours to cut the 21 trees, he has a rate of 3 trees per hour.
Helen needs only 3 hours, so she cuts 7 trees per hours.
Since they are approching, we must use the distance gap (21 trees) over their rates combined which is 10.
21/10 = 2,1.
Now we know that they will meet after 2 hours and 10 min.
But the question asks what is the portion of cut trees that Serguy would have done at the time they meet.
Since Serguy needs 7 hours to cut the 21 trees, he only did 2,1 hours for now. So the proportion is 2,1/7 which is 0,3.
I am pretty sure there is another shortcut to do this !