book Maths Puzzle Numerical Ability

What property is demonstrated in (5/9)(9/5)=1?

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I am trying to solve this problem. 54b + 2a3b. The answer that I was given was 2b(3+a)(9-3a+a2). I can get to the part of 2b(3+a)(27+a2). How do I break down (27+a2) I would n Stand and HIlda can mow the lawn in 40 min if they work together. If Hilda works twice as fast as Stan, how long does it take Stan to mow the lawn alone?