The rule is decreasing by 2.1 because when you subtract 28.9-26.8 you get 2.1 and when you subtract 26.8-24.7 you also get 2.1 The unknown term is 22.6 because 24.7-2.1 is 22.6
As you can see, the numbers after the decimal point descends by 1 point. 9 , 8 , 7 , 6 , 5 While the numbers before the decimal point descends by 2 points, 28 , 26 , 24 , 22 , 20
Notice that the box has a total os 4 + 5 + 3 = 12 balls, since there are 4 that are yellow, then, the probability to randomly obtaining a yellow ball in a single draw is:
