Another approach is to assign a "nice" value to the volume of the tank (a value that works with 1 hour and 2 hours)
Let's say the tank contains 6 gallons of water


One pipe can fill a tank in 1 hour
So it can fill 6 gallons in 1 hour.
In other words, this pipe's RATE = 6 gallons per hour

Another pipe can fill the same tank in 2 hours
If this pipe can pump 6 gallons in 2 hours, its RATE = 3 gallons per hour

How long will it take to fill the tank if both pipes are used together?
The COMBINED RATE of the two pipes = 6 + 3 = 9 gallons per hour

Time = output/rate
= 6/9 hours
= 2/3 hours

Answer: A

Cheers,
Brent