Writing and Solving a Differential Equation In Exercises 19 and $20,$ write and solve the differential equation that models the verbal statement. Evaluate the solution at the specified value of the independent variable.
The rate of change of $N$ is proportional to $N .$ When $t=0$ , $N=250,$ and when $t=1, N=400 .$ What is the value of $N$ when $t=4 ?$