Answer
120 minutes
Work Step by Step
When working together, it takes them 40 mins to mow the lawn, so they complete $\frac{1}{40}$ of the lawn in 1 minute. Let the amount of time Hilda takes be $x$. The amount of time Stan takes is $2x$. Since Hilda completes $\frac{1}{x}$ of the lawn in 1 minute and Stan completes $\frac{1}{2x}$ of the lawn in 1 minute, they complete $\frac{3}{2x}$ of the lawn in 1 minute working together. Since we know they complete $\frac{1}{40}$ of the lawn in 1 minute, we know $\frac{3}{2x}=\frac{1}{40}$. We can then determine that $x=60$ so $2x = 120$, which is the amount of time it takes for Stan to mow the lawn alone.
