Since 15 out of every 75 fish are more than 30 cm long, the probability of finding a fish more than 30 cm is P(x > 30) = 15/75 = 1/5.
The probability of finding a fish 30 cm or less is P(x ≤ 30) = 1 - P(x > 30)
= 1 - 1/5
= (5 - 1)/5
= 4/5
Since we have 800 fish and the probability of finding a fish 30 cm or less is P(x ≤ 30) = 4/5, then the number of fish in the population that are 30 cm or less in length are 800 × P(x ≤ 30) = 800 × 4/5
= 160 × 4
= 640
So, 640 fish in the population are 30 cm or less in length
