I'm only answering this one because I like the Three Stooges.
What's important in these questions is the rates, work per unit time, add up when we have multiple people.
Let c be Curly's rate in elephants per minute. Larry's rate is 1/10 elephants per minutes and Moe's is 1/6 elephants per minute. For the three of them to get done in three minutes means their collective rate is 1/3 elephants per minute:
[tex]1/3 = 1/10 + 1/6 + c[/tex]
[tex]c = -1/10 - 1/6 + 1/3 = -3/30 - 5/30 + 10/30 = 2/30 = \dfrac 1 {15}[/tex]
Curly's rate is 1/15 th of an element per minute, so it would take him 15 minutes to paint an elephant.
