উত্তর সিলেক্ট করলে সঠিক ও ভুল দেখাবে।
ব্যাখ্যা ও সঠিক সমাধান
উত্তর:
After 10 minutes
Let the time be T minutes after which pipe A need to turned off. Both of the pipes remain open together only for T minutes.
Pipe A fills 1/20 of the tank in 1 minute.
Pipe B fills 1/30 of the tank in 1 minute
Together in 1 minute they fill (1/20 + 1/30 = 1/12) of the tank.
Together in T minutes they fill = T/12 of the tank
In (15 - T) minutes, pipe B alone can fill = (15 - T)/30 of the tank
Now, T/12 + (15 - T)/30 = 1
or, {15T + 6(15 - T)}/180 = 1
or, (15T + 90 - 6T) / 180 = 1
or, 9T = 180 - 90
or, 9T = 90
So, T = 10 minutes