1. Two pipes A and B can fill a tank in 20 and 30 minutes respectively. If both the pipes are used together, then how long it will take to fill the tank ? A) 10 mins B) 12 mins C) 15 mins D) 20 mins Ans. B Part filled by A in 1 min = 1/20 Part filled by B in 1 min = 1/30 Part filled by (A+B) in 1 min = 1/20 + 1/30 = 1/12 So both pipes can fill the tank in 12 mins. 2. A cistern can be filled by a tap in 4 hours while it can be emptied by another tap in 9 hours. If both the taps are opened simultaneously, then after how much time cistern will get filled ? A) 7 hours B) 7.1 hours C) 7.2 hours D) 7.3 hours Ans. C Filled in 1 hour = 1/4 Empties in 1 hour = 1/9 Net filled in 1 hour = 1/4 - 1/9 = 5/36 So cistern will be filled in 36/5 hours i.e. 7.2 hours Comment on this question 3. A tap can fill a tank in 6 hours. After half the tank is filled then 3 more similar taps are opened. What will be total time taken to fill the tank completely. A) 2 hours 30 mins B) 2 hours 45 mins C) 3 hours 30 mins D) 3 hours 45 mins Ans. D Half tank will be filled in 3 hours Lets calculate remaining half, Part filled by the four taps in 1 hour = 4*(1/6) = 2/3 Remaining part after 1/2 filled = 1-1/2 = 1/2 23:12::1:X=>X=(12*1*32)=>X=34hrs=45 mins23:12::1:X=>X=(12*1*32)=>X=34hrs=45 mins23:12::1:X=>X=(12*1*32)=>X=34hrs=45 mins Total time = 3 hours + 45 mins = 3 hours 45 mins 4. A water tank is two-fifth full. Pipe A can fill a tank in 10 minutes and pipe B can empty in 6 minutes. If both the pipes are open, how long will it take to empty or fill the tank completely ? A) 6 min to empty B) 7 min to full C) 6 min to full D) 7 min to empty Ans. A There are two important points to learn in this type of question, First, if both will open then tank will be empty first. Second most important thing is, If we are calculating filling of tank then we will subtract as (filling-empting) If we are calculating empting of thank then we will subtract as (empting-filling) So lets come on the question now, Part to emptied 2/5 Part emptied in 1 minute = 16-110=115=>115:25::1:x=>25*15=6mins16-110=115=>115:25::1:x=>25*15=6mins16-110=115=>115:25::1:x=>25*15=6mins 5. Pipe A can fill a tank in 5 hours, pipe B in 10 hours and pipe C in 30 hours. If all the pipes are open, in how many hours will the tank be filled ? A) 2.5 hours B) 2 hours C) 3.5 hours D) 3 hours Ans. D Part filled by A in 1 hour = 1/5 Part filled by B in 1 hour = 1/10 Part filled by C in 1 hour = 1/30 Part filled by (A+B+C) in 1 hour = 15+110+130=1315+110+130=1315+110+130=13 So all pipes will fill the tank in 3 hours.