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A large cistern can be filled by 2 pipes P & Q in 120 mins and 80 mins respectively. How many mins will it take to fill the cistern from empty state if Q is used for half the time and P & Q fil it together for the other half? A) 2hrs B) 1hr C) 3hrs D) 4hrs

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A large cistern can be filled by 2 pipes P & Q in 120 mins and 80 mins respectively. How many mins will it take to fill the cistern from empty state if Q is used for half the time and P & Q fil it together for the other half? A) 2hrs B) 1hr C) 3hrs D) 4hrs
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B

Part filled by (P+Q) in 1 min = 1/120 + 1/80

=2+3/240 = 5/240

= 1/48

Suppose the cistern is filled in x mins then, x/2 (1/48 + 1/80) = 1

x/2(5+3/240)=1

8x/480 = 1

x=480/8

x= 60 mins or 1 hr
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