Geometry

In triangle $PQR$, we are given that $M$ is the midpoint of $\overline{PQ}$, $N$ is the midpoint of $\overline{PR}$, and $O$ is the intersection of $\overline{QN}$ and $\overline{RM}$. Additionally, we are told that $\overline{QN}\perp\overline{PR}$, $QN = 15$, and $PR = 20$.

Since $M$ is the midpoint of $\overline{PQ}$ and $N$ is the midpoint of $\overline{PR}$, we can conclude that $MO$ is the median of triangle $PQR$. Therefore, $MO$ passes through the midpoint of $\overline{QR}$.

Since $\overline{QN}\perp\overline{PR}$, triangle $PQN$ is a right triangle. According to the Pythagorean theorem, we have:

𝑃𝑄2=𝑄𝑁2+𝑃𝑅2PQ2=QN2+PR2 𝑃𝑄2=152+202PQ2=152+202 𝑃𝑄2=225+400PQ2=225+400 𝑃𝑄2=625PQ2=625 𝑃𝑄=25PQ=25

Since $M$ is the midpoint of $\overline{PQ}$, we have $PM = MQ = \frac{1}{2}PQ = \frac{1}{2} \times 25 = 12.5$.

Now, since $MO$ is the median of triangle $PQR$, we have $MO = \frac{1}{2}QR$. Therefore, $QR = 2 \times MO$.

To find $MO$, we can use the fact that $MO$ is perpendicular to $QN$, so triangles $QMO$ and $QNO$ are similar triangles. Therefore:

𝑀𝑂𝑄𝑁=𝑄𝑁𝑄𝑅QNMO​=QRQN​ 𝑀𝑂15=15𝑄𝑅15MO​=QR15​ 𝑀𝑂=152𝑄𝑅MO=QR152​ 𝑀𝑂=225𝑄𝑅MO=QR225​

Since $MO = \frac{1}{2}QR$, we can set up the following equation:

12𝑄𝑅=225𝑄𝑅21​QR=QR225​

Solving for $QR$, we get:

𝑄𝑅2=450QR2=450

𝑄𝑅=450QR=450​

𝑄𝑅=1510QR=1510​

Therefore, the length of $QR$ is $15\sqrt{10}$.