Wednesday, August 17, 2011

Solution for probability, Problem for rate

I'm sorry for not posting in awhile. I went on a family reunion to Yosemite for a week. It was nice :) Also, I'm not sure I'll be able to post as often as I have before, since school has started and I may be busy with homework.

Thanks everyone for trying out the problem and posting in the comments!

At a competition with N players, the number of players given elite status is equal to 21+⌊log2(N-1)⌋ - N. Suppose that 19 players are given elite status. What is the sum of the two smallest possible values of N?

We are given the equation  21+⌊log2(N-1)⌋ - N for the number of players given elite status and we are also told that 19 players have been given elite status. Thus we can make  21+⌊log2(N-1)⌋ - N equal to 19.

 21+⌊log2(N-1)⌋ - N = 19

Now we can simply it a little bit.

21+⌊log2(N-1)⌋ - N
21*2⌊log2(N-1)⌋ - N = 19
2⌊log2(N-1)⌋ - N/2 = 19/2
2⌊log2(N-1)⌋ = (19 + N)/2

Now we can put this equation into log form.

2⌊log2(N-1)⌋ = (19 + N)/2
log2((19+N)/2) = ⌊log2(N-1)⌋

Because the ⌊log2(N-1)⌋ has to be an integer (because ⌊x⌋ is the nearest integer ≤ x) we can say that log2((19 + N)/2) also has to be an integer, because they are equal to each other. Now we can start plugging in values of N that are greater than 19 (there are at least 19 players because 19 of them were given elite status). From the log equation we have (19 + N)/2 = 2x. Because the N has to be greater than 19 the lowest value 2x can be is 32. (Try out 16, you can see that it is too small)

(19 + N)/2 = 32
19 + N = 64
N = 45

We have to find the two lowest values of N so now we make 2x equal to 64

(19 + N)/2 = 64
19 + N = 128
N = 109

We can see that the 2 lowest values of N are 45 and 109. The sum of those two numbers is 154, thus the answer is 154.

Here's the next problem
A canoeist paddled upstream for 2 hours, then downstream for 3. The rate of the current was 2 mph. When she stopped, the canoeist realized she was 20 miles downstream form her starting point. How many hours will it take her to paddle back to her starting point?

Good luck! and remember, no calculators are allowed. :)


  1. Thanks for the answer. I was going crazy trying to figure it out.

  2. My brain can't cope with maths any more ;[

  3. got the answer. the answer is.... no, i'll wont tell everyone else the answer ;P

  4. Is it 4 hours? I really have trouble with math like this in my head. I'll be checking back :)

  5. I'm going to need to think about it for a little, got the last one wrong.

  6. Lol, simple problem. What is your current grade? In my country (Vietnam), we learn this shit in highschool freshman year.

  7. There is a reason I'm becoming a nurse, a limited necessity for math and statistics.

  8. This question seems a bit easier than the others. Let me just grab my calculator.

  9. I tried the last one, but seriously I HATE LOG!
    I´ll check the solution now, even hating this shit I HAVE to know. lol

  10. I'll still never understand why calculators aren't allowed