His salad calls for 8 cups of salad to maintain the given ratio. So the ratio is 5/8 for the strawberries.
Let x be what he needs for 1 cup of fruit
5/8 = x / 1 Cross multiply
5 = 8x Divide by 8
x = 5/8 of a cup of strawberries or
x = 0.625 cups of strawberries.