The easiest and systematic way of answering this type of problem is through dimensional analysis. First we need to note that each hour has 60 minutes. Each minute has 60 seconds. First multiply 15 hours with the factor 60 mins/ hour, canceling the unit hour. Then multiplying with the factor 60 sec/mins, to cancel the mins unit. You are now left with the unit of sec. The answer is 54, 000 seconds.