title: Combinations and Permutations
body: <p>A certain classroom has two rows of seats. The front row contains 8 seats and the back row contains 10 seats. How many ways are there to seat 15 students if a certain group of 4 of them refuses to sit in the back row and if a certain group of 5 others refuses to sit in the front row?</p>
  
  <p>My approach:
  4 has to go front and 5 has to go back.
  So I splited them into 4 groups</p>
  
  <pre><code>1) 4 front 4 others / 5 back 2 others
  2) 4 front 3 others / 5 back 3 others
  3) 4 front 2 others / 5 back 4 others
  4) 4 front 1 others / 5 back 5 others
  </code></pre>
  
  <p>However, I cannot put them into equations.</p>
  
  <p>Additionally, if anyone knows website that has many combination problems with detailed solutions, please let me know. Websites that I found has only very basic information.</p>
  
  <p>Thanks in advance.</p>