Pipe A can fill a tank in 45 hrs and pipe B can fill it

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Pipe A can fill a tank in 45 hrs and pipe B can fill it in 36 hrs. If both the pipes are opened in the empty tank. In how many hours will it be full?

1. 10 hr
2. 15 hr
3. 20 hr
4. 28 hr

Correct Option: C

Part filled A in 1 hr= (1/45)
Part filled B in 1 hr= (1/36)
Part filled by (A+B) together in 1 hr=(1/45)+(1/36)=1/20

So, The tank will be full in 20 hr.