Answer: it will take them 2.22 hours working together.
Explanation:
It takes Destiny y 4 hours to file all the daily paperwork. This means that unit rate of Destiny would be
1/4 per hour.
It takes Susan 5 hours to file the same paperwork. This means that unit rate of Susan would be
1/5 per hour.
If they work together, they would work at their own individual rates. This means that their individual rates are additive.
Let t represent the time it will take them if they worked together. Since their combined rate is additive Therefore,
1/4 + 1/5 = 1/t
9/20 = 1/t
t = 20/9 = 2.22 hours