Solve the Problem $a = 0.65 \sin \left( \frac { 2 \pi } { 5 } t \right)$

Essay

Solve the problem.
-Before exercising, an athlete measures her air flow and obtains $a = 0.65 \sin \left( \frac { 2 \pi } { 5 } t \right)$ where a is measured in liters per second and t is the time in seconds. If a > 0, the athlete is inhaling; if a < 0, the
athlete is exhaling. The time to complete one complete inhalation/exhalation sequence is a respiratory cycle.
Find the values of t for which the athlete's air flow is zero. Find all values of t for t < 20 seconds.