Analyzing problems involving definite integrals

The water level under a bridge is changing at a rate of $r(t)=40\sin \left({\displaystyle \frac{\pi t}{6}}\right)$‍ centimeters per hour (where $t$‍ is the time in hours). At time $t=3$‍, the water level is $91$‍ centimeters. By how much does the water level change during the $4^{\text{th}}$‍ hour?