How long does it take Stan to mow the lawn alone

Stan and Hilda can mow the lawn in 30 min if they work together. If Hilda works twice as fast as Stan, how long does it take Stan to mow the lawn alone?

Answer in minutes.

Hilda works twice as fast as Stan, so in other words, she gets twice the amount of work done in the same time.
If both mow for 30 min, the total amount of work is Stan's plus twice Stan's, which is Stan's times three.
If it takes 30 min to do Stan's work times three, it takes 30 min times three to do Stan's work.

And that's the answer: 90 min.

30=x+x/2
(30=x+x/2)2
60=2x+x
60=3x
x=20

x being the time of slower x/2 being time of the faster

since it is just a math problem and percentage of work done is not stated it is assumed they do half each in x+x/2
then it will be x+x if it is only done by the slower person
so 20+20=40

answer is 40