<p>Use nCr when order does not matter. Use nPr when order matters.</p>
<p>In other words, when you’re picking the trainees, it doesn’t matter if you pick Tom first and Jerry second, or Jerry first and Tom second. Either way, you’ll get Tom and Jerry on the job.</p>
<p>Other question: try sketching it. (the answer is 4/5, btw - flipping across the x-axis is equivalent to multiplying by -1.)</p>