Let X be the weight of fish in the lake.
Given that the mean and standard deviation as 20 and 6 respectively,

Consider the formula,

The proportion of fish weighing less than 15 pounds is calculated as,
![\begin{gathered} P(X<15)=P(z<(15-20)/(6)) \\ P(X<15)=P(z<-0.83) \\ P(X<15)=P(z<0)-P(-0.83From the Standard Normal Distribution Table,[tex]\emptyset(-0.83)=0.2967]()
Substitute the value,

Mulyiply by 100 to get the equivalent percent,

Thus, 20.33% of fish in the lake will weigh less than 15 pounds.