Step-by-step explanation
Since working alone, Kevin stuffs 1000/4 = 250 shrimps/h
Then, when his sister helps him, they both stuffs 1000/3 = 333.3 shrimps/h
Therefore, the sister stuffs 333-250 = 83 shrimps/h
Now, we can compute the sister can stuff 500shrimps in 500/83 = 6 hours
In conclusion, the solution is 6 hours.