Using the work-rate formula (not sure whether it’s taught explicitly here). notice that
\frac{1}{p} + \frac{1}{r} + \frac{1}{t} = \frac{1}{5}
and that
\frac{1}{p} + \frac{1}{r} = \frac{1}{7}
Substituting the second to the first,
\frac{1}{7} + \frac{1}{t} = \frac{1}{5}
Simplifying,
\frac{1}{t} = \frac{1}{5} - \frac{1}{7} = \frac{2}{35}
t = \frac{35}{2} = 17.5 \text{ hours}
Thanks for the answer