diff --git a/study/001_introduction-to-probability-statistics-and-random-processes/ch2/notes.ipynb b/study/001_introduction-to-probability-statistics-and-random-processes/ch2/notes.ipynb
index b7e019d..47264b4 100644
--- a/study/001_introduction-to-probability-statistics-and-random-processes/ch2/notes.ipynb
+++ b/study/001_introduction-to-probability-statistics-and-random-processes/ch2/notes.ipynb
@@ -57,9 +57,9 @@
     "\n",
     "- **ordered sampling with replacement:** $n^k$\n",
     "\n",
-    "- **ordered sampling without replacement:** $n$ choose $k$ $\\quad$ ie $\\binom{n}{k} = \\frac{n!}{k!(n-k)!}$\n",
+    "- **ordered sampling without replacement:** $n$ permute $k$ $\\quad$ ie $P^n_k = \\frac{n!}{(n - k)!}$\n",
     "\n",
-    "- **unordered sampling without replacement:** $n$ permute $k$ $\\quad$ ie $P^n_k = \\frac{n!}{(n - k)!}$\n",
+    "- **unordered sampling without replacement:** $n$ choose $k$ $\\quad$ ie $\\binom{n}{k} = \\frac{n!}{k!(n-k)!}$\n",
     "\n",
     "- **unordered sampling with replacement:** $\\binom{n + k - 1}{k}$"
    ]