<p>For the first question:</p>
<p>Since there are 4 experience plumbers and 1 spot, there are 4 possibilities of plumbers.</p>
<p>Since there are 2 spots for 4 trainees, and order does not matter, use combinations. (Use combinations whenever you are choosing members for a team or group because order will not matter). The combination will be 4 C 2 = 6. That just means 4 trainees for 2 spots.</p>
<p>Using the multiplication counting principle, you will do 4 x 6 = 24</p>