A tank can be filled by a tap in 20 minutes and by another tap in 60 minutes. Both the taps are kept open for 10 minutes and then the first tap is shut off. After this, the tank will be completely filled in what time ?
Choose the correct choice from the following choices.
A) 10 mins  B) 15 mins  C) 20 mins  D) 25 mins  E) 30 mins

Rationale: explanation : how we can solve this question ? first we will calculate the work done for 10 mins , then we will get the remaining work , then we will find answer with one tap work , as part filled by tap a in 1 min = 1 / 20 part filled by tap b in 1 min = 1 / 60 ( a + b ) ' s 10 mins work = 10 ∗ ( 1 / 20 + 1 / 60 ) = 10 ∗ 4 / 60 = 2 / 3 remaining work = 1 − 2 / 3 = 1 / 3 method 1 = > 1 / 60 : 1 / 3 = 1 : x = > x = 20 method 2 1 / 60 part filled by b in = 1 min 1 / 3 part will be filled in = 1 / 3 / 1 / 60 = 60 / 3 = 20 option c
Answer: C