A bag containing 5 red balls, 8 blue balls and 7 yellow balls, what % of the balls are blue?

5+7+8=20 which is the total number of balls that the bag contains

because out of the 20 balls only the 8 are blue, it is right to say that

8/20 = x/100 ⇒ x = (8·100) / 20 ⇒ x = 800/20 ⇒ x=40

the blue balls are the 40%