3o L--------------- 12 sec
800 L ------------- x
x=800x12:30=320 sec
30L------------------- 12 sec
1000 L ------------------- x
x=1000x12:30=400 sec
30 L------- 12 sec
x L-------------- 10 sec
x=10x30:12=25 L
30L -------------- 12 sec
x ------------------------ 18 sec
x=30x18:12=45 L
30 L--------------- 12 sec
x ------------------- 120 sec
x=30x120 : 12=300 L