#### Problem 96E

96. The biomass $B(t)$ of a fish population is the total mass of the members of the population at time $t$. It is the product of the number of individuals $N(t)$ in the population and the average mass $M(t)$ of a fish at time $t$. In the case of guppies, breeding occurs continually. Suppose that at time $t=4$ weeks the population is 820 guppies and is growing at a rate of 50 guppies per week, while the average mass is $1.2 \mathrm{~g}$ and is increasing at a rate of $0.14 \mathrm{~g} /$ week. At what rate is the biomass increasing when $t=4 ?$