The first tap takes 5 hours to complete the job, so it fills 1/5 of the pool per hour.
The first tap takes 7 hours to complete the job, so it fills 1/7 of the pool per hour.
Together, they fill
[tex]\dfrac{1}{5}+\dfrac{1}{7}=\dfrac{7+5}{35}=\dfrac{12}{35}[/tex]
of the pool per hour, implying that they fill the pool in 35/12 hours
