title: String replacement in python is replacing the entire string
body: <p>When I do the string replacement I am getting an error. For example: my string is <code>my_string = '15:15'</code>. I want to replace <code>15</code> which is after the colon to <code>30</code>. For example I need <code>'15:30'</code>. When I try to do the string replace it's working fine for all other values for example, <code>'09:15'</code>, <code>'09:20'</code>.</p>
  
  <p>I have tried:</p>
  
  <pre><code>my_string = '15:15'
  my_new_string = my_string.replace(my_string[-2:], '30')
  
  my_string = '15:15'
  my_new_string = my_string.replace(my_string[-2:], '30')
  </code></pre>
  
  <p>What I am expecting is <code>15:30</code> but my actual output is <code>30:30</code></p>