A tank can be filled by pipe A in 5 hours and by pipe B in 8 hours, each pump working on its own. When the tank is full and a drainage hole is open, the water is drained in 20 hours. If initially the tank was empty and someone started the two pumps together but left the drainage hole open, how long does it take for the tank to be filled

Step-by-step explanation:

tank filled in 1 hour by pipe A=1/5

tank filled in 1 hour by pipe B=1/8

water drained out in 1 hour=1/20

tank filled in 1 hour by all three=1/5+1/8-1/20=(8+5-2)/40 =11/40 [L.C.M. of 5,8,20=40]

time taken to fill the tank=40/11=3 7/11 hours

question: how many people can she serve?

answer: she can serve 16.66666667 people. so she can only give 16 people a complete serving.

question: does she have enough ice cream for her guests?

answer: no. explanation: she only has enough for 16 people to have a full serving. however, she can give a 17th person a partial serving.