Answer:

Step-by-step explanation:
Let third cook take x hours to prepare the same number of  pies y alone
Let y be the number of pies
In 1 hour ,third cook prepare pies=
One experienced cook takes time to prepare enough pies  =4 hours
In 1 hour , cook prepare pies=
Another cook takes time to prepare same number of pies =7 hours
In 1 hour , another cook prepare pies=
All three cooks take time to  prepare the same number of pies=2 hours
In 1 hour,all three cooks  prepare  pies=
According to question 





 hours
 hours
Hence, the third cook takes time to prepare the same number of pies alone=