A winery has a barrel to hold chardonnay. An inlet pipe can fill the barrel in 18 hours, while an outlet pipe can empty the barrel in 21 hours. How many hours would it take to fill the barrel if both the inlet and outlet pipes are open

Aclassroom is made up of 11 boys and 14 girls. the teacher has four main classroom responsibilities that she wants to hand out to four different students (one for each of the four students). if the teacher chooses 4 of the students at random, then what is the probability that the four students chosen to complete the responsibilities will be all boys?