Answer
$17.5$ hours
Work Step by Step
Say that I need x hours to finish the job on my own.
In 1 hour, I do $\displaystyle \frac{1}{x}$ of the full job, and
my cousin does $\displaystyle \frac{1}{7}$ of the full job.
In 5 hours , we complete $(\displaystyle \frac{5}{x}+\frac{5}{7})$ of the job.
The text says that this is $\displaystyle \frac{1}{1}$ of the job (the whole job).
$\displaystyle \frac{5}{x}+\frac{5}{7}=1\qquad$...multiply with $7x$
$ 35+5x=7x\qquad$... subtract $5x$
$ 35=2x\qquad$... divide with 2
$x$= $17.5$ hours.
