GMAT Quantitative Reasoning Practice Test 3

The output of Avery is $\frac{1}{3}$ walls in one hour and the output of Tom is $\frac{2}{5}$ walls in one hour.
The two worked together for one hour, their combined output is $\frac{1}{3}+\frac{2}{5}=\frac{11}{15}$ wall and that's the fraction of the wall that they completed together. Tom has $1-\frac{11}{15}=\frac{4}{15}$ wall left, with his current output it will take him $\frac{\left(\frac{4}{15}\right)}{\left(\frac{2}{5}\right)}=\frac{2}{3}$ hours, which is 40 minutes.