Help with a question

GRE Quant Problem Solving
1

Pump A can empty a pool in A minutes and pump B can empty the same pool in B minutes. Pump A begins emptying the pool for 1 minute before pump B joins. Beginning from the time pump A starts, how many minutes will it take to empty the pool?

                                                                                                                         SOURCE- YOUTUBE
2

Work = Rate*Time

Pump A:
1 pool = Rate(A)*A mins
Rate(A) = 1/A

Similarly:
Rate(B) = 1/B

Amount emptied by A in 1 min = Rate(A)*1min = 1/A

Amount to be emptied after 1 min: 1 - (1/A)

working together: 1 - (1/A) = ( Rate(A) + Rate(B) )*Combined_time where Rate(A) and Rate(B) can be substituted from above.

Final time: 1 min + Comined_time

3

It’s from Magoosh - see Pump A can empty a pool in A minutes, and pump B can empty : Problem Solving (PS).

In future, link the video - “YouTube” is too generic.