Answer:
[tex]7[/tex] times.
Step-by-step explanation:
We know that there are three buildings that he can be assigned to.
Since there are three buildings, the probability of each crew would be [tex]\frac{1}{3}[/tex].
If you set up the proportion, it would look like this.
[tex]\frac{1}{3}[/tex] = [tex]\frac{x}{21}[/tex]
The next step would be to solve for x.
[tex]3x =21[/tex]
[tex]x=7[/tex]