While baking, evan discovered a recipe that required 1/2 cups of walnuts for very 2 1/4 cups of flour. how many cups of walnuts will he need for 4 cups of flour.

9/4 cups of flour is equal to 2 1/4 2/4 cups of flour is equal to 1/2 SO 16/4 cup flour is equal to 4 Which is 7/4 cups more than 2 1/4 7/4 cups is 1 3/4 cups (1/2 a cups less than 2 1/4) Which means he needs 1 3/4 cups of walnuts. 
Simplfied- 4 cups is 1/2 a cups short of double, since the flour is done is quarters, take the 1/4 cup away from the walnuts. 
(times 1/2 by 1/2 you get 1/4 subtract it)