diff --git a/15/b/4.py b/15/b/4.py
new file mode 100644
index 0000000..da0382c
--- /dev/null
+++ b/15/b/4.py
@@ -0,0 +1,7 @@
+def choose(n, k):
+   res = n/k
+   while k > 1:
+      n-= 1
+      k -= 1
+      res *= n/k
+   return res