A tank can be filled by pipe A in 3 hours and Pipe B in 5 hours. At 7 am, pipe A was opened. At what time will the tank be filled if pipe B was opened at 8 am?

9 : 30 am

9 : 15 am

9 : 45 am

None of these

Pipe A and pipe B can fill the tank in 3 hours and 5 hours, then total work = LCM of (3, 5) = 15 unit

Efficiency of pipe A = 15/3 = 5 unit and efficiency of pipe B = 15/5 = 3 unit

From 7 to 8 AM, pipe A alone done 5 unit, work remains (15 - 5) = 10 unit,

Now, pipe A and B together can do it in = 10/(5 + 3) = 10/8 hours = 10/8 × 60 = 75 min

The required time is 8 AM + 75 min = 9 AM + 15 min = 9 : 15 AM

∴ The correct answer is 9 : 15 AM