Machine A can do a job 50% faster than machine B. When they work together, they can complete a job in 4 hours. Approximately, how long does it take Machine A to complete the job alone?

I read on gmatclub that : *ratio of time taken will be inverse of the ratio of rate of work since work done in both the cases is the same*

So here :

rA /rB = 1.5/1 = 3/2

Time would be inverse so: tA/tB = 2/3

tA/(tA+tB) = 2/5

2/5 = tA/4

so tA=8/5 hours but the answer is 20/3 hours. Could someone explain where I went wrong? Did I misunderstand the inversion rule b/w rate and time?