Olympiad Maths Prep

Track / Stage 3 / 187 of 260 #187 of 2000

Problem 187

AMC 10/12, early questions
Algebra Difficulty 3.7 Find the answer

AA consists of mm consecutive integers whose sum is 2m2m, and set BB consists of 2m2m consecutive integers whose sum is m.m. The absolute value of the difference between the greatest element of AA and the greatest element of BB is 9999. Find m.m.

Official solution

Note that since set AA has mm consecutive integers that sum to 2m2m, the middle integer (i.e., the median) must be 22. Therefore, the largest element in AA is 2+m122 + \frac{m-1}{2}.
Further, we see that the median of set BB is 0.50.5, which means that the "middle two" integers of set BB are 00 and 11. Therefore, the largest element in BB is 1+2m22=m1 + \frac{2m-2}{2} = m. 2+m12>m2 + \frac{m-1}{2} > m if m<3m < 3, which is clearly not possible, thus 2+m12<m2 + \frac{m-1}{2} < m.
Solving, we get \begin{align*} m - 2 - \frac{m-1}{2} &= 99\\ m-\frac{m}{2}+\frac{1}{2}&=101\\ \frac{m}{2}&=100\frac{1}{2}.\\ m &= \boxed{201}\end{align*}

Source: NuminaMath-1.5, licensed Apache-2.0. Statement and solution reproduced as published; topic, difficulty and ordering added by this site.