Ritu's chemistry practical begins at the morning time on clock A and ends at the time on clock B. How long is the practical?

A shows 9:20 and B shows 11:05. From 9:20 to 10:20 is 1 h, and from 10:20 to 11:05 is 45 min, so 1 h 45 min. (2 h 15 min subtracts 20 from 05 the wrong way round.)