math
posted by sam .
In 6 hours,an experienced cook prepares enough pies to supply a local restaurant's daily order. Another cook prepares the same number of pies in 7 hours. Together with a third cook, they prepare the pies in 2 hours. Find how long it takes the third cook to prepare the pies alone.

if the 3rd cook takes x hours,
1/2 = 1/6 + 1/7 + 1/x
x = 21/4 = 5 1/4