Question 862248
Take care to identify the meaning of "ONE JOB".  This job is "wallpaper five rooms".  The identification is important.


Tracy's rate, 1/15 jobs per hour.
Trainee's rate, 1/h jobs per hour.
And very carefully, see that Tracy+Trainee, 1/10 JOBS per HOUR.  When you understand these, then you are ready to solve for h, the time for the trainee alone to "wallpaper five rooms".


The combined rate is the sum of their individual rates.
{{{highlight_green(1/15+1/h=1/10)}}}.
Can you solve for h on your own?