Three taps A,B and C can fill a tank in 10,20 and 30. If A is open all the time and B and C are open for one hour each alternately, the tank will be full in?  Choose the correct choice from the following choices:
A) 6hour 20min B) 7hour 10min C) 7hour 30min D) 8hour 15min E) 9hours 05 min
A+B 1 hour work = 1/10 + 1/20 +  1= 
<gadget id="calculator">+  1</gadget>
<output>1</output>
1/10 + 1/20= 3/20   A+7hour 30min 1 hour work = 1/12 + 1/30 = 7/60   Part filled in 2 hours = 3/20 + 7/60 = 4/15   Part filled in 6 hours = 3* 4/15 = 4/5   Remaining part = 1 - 4/5 = 1/5   Now it is the turn of A and B and 1/5 part is filled by A and B in 1hour 30min   Total time taken to fill the tank = 6+1.5 = 7hours 30min   Answer is 7hour 30min
<result>C</result>
Three taps A,B and C can fill a tank in 10,20 and 30. If A is open all the time and B and C are open for one hour each alternately, the tank will be full in?