To find out how many cups of borax and washing soda combined Lenny needs, we need to calculate the total number of parts in the recipe.
The recipe calls for 2 parts borax, 2 parts washing soda, and 1 part soap flakes.
Since borax and washing soda have the same proportion, we can combine their parts together. So the total number of parts for borax and washing soda is 2 + 2 = 4 parts.
To find out how many cups of borax and washing soda combined Lenny needs, we need to divide the total number of cups of soap flakes by the total number of parts.
So, 7 cups / 4 parts = 1.75 cups.
Therefore, Lenny needs a total of 1.75 cups of borax and washing soda combined.