If 3 taps are opened together a tank is filled in 3 hrs. One of the taps alone can fill it in 10 hrs. and another in 15 hrs. In how many hours will the third tap fill it ?
Options: A) 7 hrs . B) 4 hrs . C) 5 hrs . D) 6 hrs . E) 8 hrs .

Rationale: part filled by three taps together in 1 hr = 1 / 3 part filled by one of the tap in 1 hr = 1 / 10 part filled by another tap in 1 hr = 1 / 15 part filled by third tap in 1 hr = 1 / 3 - ( 1 / 0 + 1 / 15 ) = 5 / 30 therefore the third tap can fill the whole tank in : 30 / 5 hrs . = 6 hrs . answer : d
Answer: D