His salad calls for 8 cups of salad to maintain the given ratio. (5 cups of strawberries for every 3 cups of blueberries) So the ratio is 5/8 for the strawberries.
Let x be what he needs for 1 cup of fruit
5/8 = x
That's because there are 5 cups of strawberries for every 8 ( 5 strawberries + 3 blueberries ) items he needs.