15. An important model for commercial fisheries is that of Beverton and Holt. It begins with the study of a single cohort of fish- that is, all the fish in the study are born at the same time. For a cohort of the North Sea plaice (a type of flatfish), the number N = N(t) of fish in the population is given by N = 1000e-air and the weight w = w(t) of each fish is given by WE 6.32 1-.93e-(063 Here w is measured in pounds and t in years. The variable t measures the so-called recruitment age, which we refer to simply as the age. The biomass B = B(t) of the fish cohort is defined to be the total weight of the cohort, so it is obtained by multiplying the population size by the weight of a fish. If a plaice weighs 4.21 pounds, how old is it? Round our answer to the nearest year