1. if the question can be answered by using one of the statements alone, but cannot be answered using the other statement alone
" It takes 40 minutes each way and 20 minutes of waiting time at each terminal. "
this statement is required for answering question
boat ferried 1500 passengers across the river in twelve hours
40 minutes each way and 20 minutes of waiting time at each terminal
So, total time in one trip 40+40+20=100 min
In 100 min there are only 1 trip
then in 12*60=720 min there are 720/100= 7.2
i.e. 7 full trip done