The time that the tank will be full if tap B was opened 3 minutes after tap A was opened is D. 9 min 12 s
How to find the time ?
Tap A can fill the tank in 10 minutes, so R_A = 1/10 tank per minute.
Tap B can fill the tank in 15 minutes, so R_B = 1/15 tank per minute.
For the first 3 minutes, only tap A is open, so it fills:
3 minutes x R_A = 3 x (1/10)
= 3/10 of the tank
To find the remaining time to fill the tank, we can use the formula:
Time = Amount to be filled / Rate
Time = 1 / (1/6) = 6 minutes 12 seconds
Now, add the 3 minutes it took tap A to fill the initial portion:
Total time = 6 minutes + 3 minutes
= 9 minutes 12 seconds