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Official Solution:

\(x\) and \(y\) are integers from 0 to 170, both inclusive. For how many ordered pairs \((x,y)\) is the average (arithmetic mean) of \(x\) and \(y\) equal to 60 ?

A. \(60\)
B. \(118\)
C. \(119\)
D. \(120\)
E. \(121\)


The average (arithmetic mean) of \(x\) and \(y\) is 60 means that the sum of \(x\) and \(y\) is 120.

\(x+y=120\).

\(x\) and \(y\) are integers from 0 to 170, both inclusive, so \((x,y)\) can be:

\((0,120)\);

\((1,119)\);

\((2,118)\);

\((3,117)\);

\(...\)

\((120,0)\).

So, there are total of 121 such pairs.


Answer: E

Hi Bunuel, what is ordered pair? I have considered for instance, (1,119) the same as (119,1). Hence, my answer is not correct. Thanks!

An ordered pair, is a pair of elements arranged in a specific order. For example, (1, 119) and (119, 1) are considered different ordered pairs.
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from (61,59) to (120,0) i counted 60 pairs, and inverting those (59,60 to 0,120) totals to 120. I assumed 60,60 would not be included as x,y are 2 variables that cant be the same number at once. Is that line of thought incorrect?
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from (61,59) to (120,0) i counted 60 pairs, and inverting those (59,60 to 0,120) totals to 120. I assumed 60,60 would not be included as x,y are 2 variables that cant be the same number at once. Is that line of thought incorrect?

Yes, that line of thought is incorrect. Two variables can have the same value unless the question states that they must be distinct. Therefore, (60,60) is a valid ordered pair.

You counted 60 pairs on each side, and adding (60,60) gives 60 + 60 + 1 = 121 pairs.
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