It should be 13 since 13+8 = 21. 21-7 = 13
Answer: 
Step-by-step explanation:
Given
The yellow line leaves the station every 6 min
The red line leaves the station every 8 min
They both leave at 
Both trains meet again after, LCM(6,8)=24 min
i.e. at

Divide. 1/3 pounds of cheese divided by 3 sandwhiches 1/3 / 3= 1/9 pounds of cheese.
.......................................................................................