Susan can wax her car in 2 hr(s). When she works together with Helga, they can wax the car in 30 minute(s). How long would it take Helga, working by herself, to wax the car?

If you add units to this equation it will make a lot more sense.

And the solution gives an answer of x=2/3, but 2/3's what? day, hour, min

Susan is 1 car per 2 hours

Helga is 1 car per x.

Together 1 car per 2 hours + 1 car per x = 1 car per (1/2)hour

then 1 car / 2 hours + 1 car / x = 1 car / (1/2) hour = 2 cars / hour

1 car / x = 2 cars / hour - 1 car / 2 hours

= 4 cars / 2 hour - 1 car / 2 hours

= 3 cars / 2 hour

So 1 car / x = 3/2 cars/hour

Therefor x = 2/3 hours / car