Predicted waves out of 50 , caught = 4 / 5 or 20% of 50 = (4 / 5) x 50 = 40
Alternatively, multiplying numerator & denominator of wave catch probability ie 4 / 5 with 10 each , we get (4 x 10) / (5 x 10) = 40 / 50 . This also signifies 40 out of 50 waves are predicted to be caught.