Using estimates of rainfall, evaporation, and water consumption, the town engineer developed the following model of the water volume in the reservoir as a function of time

V(t)=10^9+10^8(1-e^(-t/100))-rt

where V is the water volume in liters, t is time in days, and r is the town's consumption rate in liters per day. Write two user-defined functions. The first function should define the function V(t) for use with the fzero function. The second function should use fzero to compute how long it will take for the water volume to decrease to x percent of its initial value of 10^9L. The inputs to the second function should be x and r. Test your functions for the case where x=50 percent and r=10^7 L/day.