Answer
$10$ hours
Work Step by Step
In 1 hour,
crew 1 dispenses $\displaystyle \frac{1}{20}$ of the needed supplies,
crew 2 dispenses $\displaystyle \frac{1}{30}$ of the needed supplies,
crew 3 dispenses $\displaystyle \frac{1}{60}$ of the needed supplies,
In t hours , they dispense
$(\displaystyle \frac{t}{20}+\frac{t}{30}+\frac{t}{60})$ of the needed supplies.
We want the t (in hours) when this is $\displaystyle \frac{1}{1}$ of the needed supplies.
$\displaystyle \frac{t}{20}+\frac{t}{30}+\frac{t}{60}=1\qquad$...multiply with $LCD=60$
$3t+2t+t=60 $
$ 6t=60\qquad$... divide with $10$
$x$= $10$ hours.
