The population of a community of foxes is observed to fluctuate on a 10 -year cycle due to variations in the availability of prey. When population measurements began $(t=0),$ the population was 35 foxes. The growth rate in units of foxes/ year was observed to be $$P^{\prime}(t)=5+10 \sin \left(\frac{\pi t}{5}\right)$$. a. What is the population 15 years later? 35 years later? b. Find the population $P(t)$ at any time $t \geq 0.$