Math - Help me out

Question

Working alone, it takes you 30 minutes, your friend 30 minutes, and your supervisor 15 minutes to unload a delivery truck. working together, how much time does it take all three of you to unload the truck?

Answer 1

Let's assume that they are unloading a truck with 300 boxes on it! (300 is arbitrary but makes math easy)

The first person unloads 300 boxes/30 min. second person unloads 300 boxes/30 min and the third person unloads 300 boxes/15 minutes or (all)... 10 boxes/min , 10 boxes/min, and 20 boxes/min

So, if they work together, they can unload (10+10+20)boxes/min = 40boxes/min

So, working together 300 boxes/40 boxes/min = 7.5 minutes.