Answer: In 36 minutes both buses will end up at the library. 
 
Step-by-step explanation: 
 
Given: Buses from one route stop at the library every 9 minutes. 
 
Buses from the other route stop at the library every 12 minutes. 
 
The number of minutes in which both buses will end up at the library= LCM(9,12) 
 
Now , 9= 3 x 3 
 
12= 2 x 2 x 3 
 
LCM(9,12) = 2 x 2 x 3 x 3 = 36