diff --git a/source/exercises100.ktx b/source/exercises100.ktx
index 63a8ce3..1fee155 100644
--- a/source/exercises100.ktx
+++ b/source/exercises100.ktx
@@ -778,9 +778,18 @@ hint: any, ~
 < a60
 # Author: Warren Weckesser
 
+# null : 0 
 Z = np.random.randint(0,3,(3,10))
 print((~Z.any(axis=0)).any())
 
+# null : np.nan
+Z=np.array([
+    [0,1,np.nan],
+    [1,2,np.nan],
+    [4,5,np.nan]
+])
+print(np.isnan(Z).all(axis=0))
+
 < q61
 Find the nearest value from a given value in an array (★★☆)