In 1 hour, A can mow 1/2 the lawn.
In 1 hour, B can mow 2/5 of the lawn.
Together in 1 hour, they can mow 1/2 + 2/5 = 9/10 of the lawn.
So to complete the lawn mowing, it will take them 10/9 hours.

Its rare we have a Winner before a post from Saswat