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 #1
avatar+776 
+2

I can't do it with 3 uses of saw, but I can do it with 4 uses of the see-saw. 

 

There are 12 people. A,B,C,D,E,F G H I J K, and #. Let's assume # is the odd one. Since we do not know if # is heavier or lighter than the others, this makes it a tad bit harder. 

 

1) Divide the 12 people into two groups. A - F and G - #. We measure one of the groups, 3 on one side and 3 on the other. Let's just say we measure ABC to DEF. If they are equal, that means the odd duck is in the second group. If they are not even, we go to step 2. For our purpose, let's assume Group 2 has the odd one, the #.

 

2) Choose 4 of the 6 from the group and measure them. Let's just say we measure GH and IJ. If they are equal, then the odd duck is in the group of 2. If they are not equal, then the odd duck is inside the group of 4.

 

3) a) If the group has 2 people, in our case K and #, we choose K or # and measure it with one of the normal ones. In our case, let's measure K and G. If they are equal, then the non-measured one (#) is the odd duck, but if they are not equal, then the one that was chosen to be measured with the normal one, in our case K, is the odd duck.

 

3) b) If the group has 4 people, randomly choose 2 people and take them out. Let's make a new scenario, where the group of 4 is I, J, K and #. We randomly choose out K and #. We measure I and J. If they are equal, the odd one is either K or #, which we can find by using step 2 above. If I and J are not equal, then do as above in step 2.

 

4) More of step 2. The 4th step is only needed in some scenarios. 

 

- PM

 

P.S. please send the link to the solution...I want to know a quicker way. wink

 #1
avatar+6250 
+1
Jan 6, 2019
Jan 5, 2019
 #1
avatar+6250 
0
Jan 5, 2019
 #1
avatar+24 
+1

1. Let the first term be a, and let the common difference be d. Then the four positive integers are a, a+d, a+2d, and a+3d. The sum of these four positive integers is 4a+6d=46, so 2a+3d=23. Solving for d, we find that =(23-2a)/3. The third term is a+2d=a+2(23-2a)/3=(46-a)/3. Thus, to maximize this expression, we minimize a, so a must be 1. Substituting 1 in, we find that the largest possible third term is 15.

 

2. 94 =F + 26D,  
27 =F + 93D, solve for F, D
F =120 - First term
D = -1 - Common difference

 

3.  As we begin, we note that no side of the triangle can exceed 29 units in length. Let’s start at the 20–20–20 triangle. We first count the number of triangles that have 2 sides of length less than 20. Let the third side be of length 20+k where 2≤k≤9. This we do by taking away k units in length from the first and the second sides, by subtracting at least one unit from each. For each k, We can do it in ⌊k/2⌋ ways. Thus, counting for each possible k, we have 1+1+2+2+3+3+4+4=201+1+2+2+3+3+4+4=20 cases.

Let’s now go back to the 20–20–20 triangle and count the number of cases where only one side has a length <20<20 units. We can take away 1 to 18 units from this one side and redistribute it to the other two such that no side exceeds 29 units in length. Let us take away k units from this one side, such that 1≤k≤9. In order for the triangles to be distinct, the redistribution can be done in ⌊k/2⌋+1⌊k/2⌋+1 ways. Then, for all possible values of k in this range, we have 1+2+2+3+3+4+4+5+5=29 cases. Now, let us consider the cases where 9
Finally, we add up all our cases:

1+20+29+25=751+20+29+25=75 distinct triangles. Also, subtract one because of the equilateral one.

 

4. I really can't think of another way other than listing.

Jan 5, 2019

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