Question 1115780
.
<pre>
Under the given condition, the chief can dispatch 1-st truck to any of 7 fires: 7 opportunities;

                                                  2-nd truck to any of 7 fires: 7 opportunities;

                                                  3-rd truck to any of 7 fires: 7 opportunities;

                                                  4-th truck to any of 7 fires: 7 opportunities;

. . . . . . . . . . .      and  so  on  . . . . . . . . . .  

                                                 21-th truck to any of 7 fires: 7 opportunities.


In all, there are  {{{7^21}}} different ways.
</pre>

Solved.