diff --git a/source/Collection_Data_Types.rst b/source/Collection_Data_Types.rst
index a6bce05132ecaa3c4d8f3571e3ce14ea8fa0a586..c1bcc6464c39bcd43099dcdc6f2c6020f5ce4e5d 100644
--- a/source/Collection_Data_Types.rst
+++ b/source/Collection_Data_Types.rst
@@ -105,7 +105,7 @@ wihout using python shell, what is the results of the following statements:
    sum is a function which return the sum of each elements of a list.
       
 ::
- 
+
    x = [1, 2, 3, 4]
    x[3] = -4 # what is the value of x now ?
    y = sum(x)/len(x) #what is the value of y ? why ?