2/7 of a cup
This means he needs 5/7 of the 2/5 cup of the whole recipe, so it is (5*2)/(7*5) or 10/35 or 2/7 of a cup.
The recipe need cups of flour.
Also, the chef is making the amount which is of the recipe.
Thus, the total amount of the flour required will be,
Hence, the chef should use cups of flour.