I am confused on the problems.

3. The tide removes sand from Sandy Point Beach at a rate modeled by the function R, given by R(:) = 2+5sin 25) A pumping station adds sand to the beach at a rate modeled by the function S. given by S(1)- 151 1+ 32 Both R(r) and S(1) are measured in cubic yards per hour and t is measured in hours for 0St56. At time 1 = 0, the beach contains 2500 cubic yards of sand. a. Find the rate at which the total amount of sand on the beach is changing at time t = 4. 4. The rate at which rainwater flows into a drainpipe is modeled by the function R, where cubic feet per hour, i is measured in hours, and 0Et$8. The pipe is R(t) = 20sin partially blocked, allowing water to drain out the other end of the pipe at a rate modeled by D(1) =-0.0413 +0.412 +0.96t cubic feet per hour, for 0Et 8. There are 30 cubic feet of water in the pipe at / = 0. a. Is the amount of water in the pipe increasing or decreasing at time f = 3 hours? Give a reason for you