Triple the fun ...

Algebra Level 5

Find the number of distinct ordered triples ( a , b , c ) (a,b,c) that satisfy the equation

a b c 16 = 4 a + 4 b + 4 c abc - 16 = 4a + 4b + 4c ,

where a , b , c a,b,c are positive (not necessarily distinct) integers.


The answer is 46.

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1 solution

A quick overview of one possible method ....

Let x = a + 2 , y = b + 2 x = a + 2, y = b + 2 and z = c + 2 z = c + 2 . Then the given equation becomes

1 x + 1 y + 1 z = 1 2 \dfrac{1}{x} + \dfrac{1}{y} + \dfrac{1}{z} = \dfrac{1}{2} .

Now look for solutions with 3 x 6 3 \le x \le 6 and x y z x \le y \le z . A straightforward search gives us the following triples ( x , y , z ) (x,y,z) :

( 3 , 7 , 42 ) , ( 3 , 8 , 24 ) , ( 3 , 9 , 18 ) , ( 3 , 10 , 15 ) , ( 3 , 12 , 12 ) , ( 4 , 5 , 20 ) , ( 4 , 6 , 12 ) , ( 4 , 8 , 8 ) , ( 5 , 5 , 10 ) (3,7,42), (3,8,24), (3,9,18), (3,10,15), (3,12,12), (4,5,20), (4,6,12), (4,8,8), (5,5,10) and ( 6 , 6 , 6 ) (6,6,6) .

This translates to the following triples ( a , b , c ) (a,b,c) :

( 1 , 5 , 40 ) , ( 1 , 6 , 22 ) , ( 1 , 7 , 16 ) , ( 1 , 8 , 13 ) , ( 1 , 10 , 10 ) , ( 2 , 3 , 18 ) , ( 2 , 4 , 10 ) , ( 2 , 6 , 6 ) , ( 3 , 3 , 8 ) (1,5,40), (1,6,22), (1,7,16), (1,8,13), (1,10,10), (2,3,18), (2,4,10), (2,6,6), (3,3,8) and ( 4 , 4 , 4 ) (4,4,4) .

Factoring in all distinct permutations of these 10 10 triples gives us a final number of 46 \boxed{46} ordered triples satisfying the original equation.

wasn't able to do level 1 problems NOW I DID LEVEL 5 PROBLEM VOW!!!

Manit Kapoor - 6 years, 8 months ago

I keyed in the answer only once and the correct answer and I was told wrong.

Chew-Seong Cheong - 6 years, 9 months ago

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That's strange. You should let @Calvin Lin know in case there is a glitch in the system. He has a messageboard here ; just post your concern and he should get back to you in a day or two.

Brian Charlesworth - 6 years, 9 months ago

Can you state the list of events? What did you try entering?

I do not see that you entered any value. Note that you should get 3 tries, unless you clicked on "Reveal Solutions".

Calvin Lin Staff - 6 years, 9 months ago

Calvin/Brian, I just keyed in the answer right for the first time. I keyed in and it shows Correct! When I came in to see the solution and went out and back again. It shows I did not solve the problem and I did not see my solution appeared on top. Then, I realized there were two other questions, I tried earlier had the same problem too.

Chew-Seong Cheong - 6 years, 9 months ago

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I hope Calvin can sort things out for you. I had a weird thing just happen as well, where I answered a question correctly, but for the first few seconds after answering the right-hand side of the screen went blue, saying that I did not solve the problem, then finally turned green, indicating that I had solved the problem. It was a multiple-choice question, but your problems have been with 'three try' questions, so the glitch seems to be acting indiscriminately.

Brian Charlesworth - 6 years, 9 months ago

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It happened to me once also on a multiple choice question, yesterday. I have answered two questions correctly today. So I didn't lose the streak. Because the earlier one I solved was yesterday, US time (12 hours apart). The first one registered correctly, the second one did not. The difference was whether the dialog box inviting for adding or voting a solution appears. If it appears it is okay. I think it is connection problem. I am using wifi at home. Very often, I can't get Brilliant website online. I will use my desktop in the future. Now, as I think of it, it first happened to me three months ago, that was when I lost my previous streak of 40+ days. I remembered very well that I had solved the problem for that day.

Chew-Seong Cheong - 6 years, 9 months ago

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@Chew-Seong Cheong Yes, it does sound as if it is more of a connection problem. I see that you are closing in on a 100-day streak; wouldn't want to break that because of a bad connection.

Brian Charlesworth - 6 years, 9 months ago

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