Answer
$\displaystyle \frac{60}{13}\approx 4.6$ hours.
Work Step by Step
In 1 hour,
crew 1 dispenses $\displaystyle \frac{1}{10}$ of the needed supplies,
crew 2 dispenses $\displaystyle \frac{1}{15}$ of the needed supplies,
crew 3 dispenses $\displaystyle \frac{1}{20}$ of the needed supplies,
In t hours , they dispense
$(\displaystyle \frac{t}{10}+\frac{t}{15}+\frac{t}{20})$ of the needed supplies.
We want the t (in hours) when this equals $\displaystyle \frac{1}{1}$ of the needed supplies.
$\displaystyle \frac{t}{10}+\frac{t}{15}+\frac{t}{20}=1\qquad$...multiply with $LCD=60$
$6t+4t+3t=60$
$ 13t=60\qquad$... divide with $13$
$t$= $\displaystyle \frac{60}{13}\approx 4.6$ hours.
