You can see from the second example the order is left to right:

queueTime([10,2,3,3], 2)
// should return 10
// because here n=2 and the 2nd, 3rd, and 4th people in the 
// queue finish before the 1st person has finished.

I was confused by this as well! The queue reads left-to-right and not right-to-left.

The front person in the queue (i.e. the first element in the array/list) proceeds to a till as soon as it becomes free.

solution in 1 string :0

2 and 3 go to different tills. Read the description again.

why
queue_time([2,3,10], 2)

should return 12

first checkout working 2 + 3 == 5 times
and
second checkout working 10 times
10 biggest than 5 and should give 10. but correct answer is 12 :/