Answer
$t=6$ hours
Work Step by Step
In 1 hour,
pipe 1 fills $\displaystyle \frac{1}{8}$ of the (full) pool,
pipe 2 fills $\displaystyle \frac{1}{24}$ of the pool.
In $t$ hours, they fill $(\displaystyle \frac{t}{8}+\frac{t}{24})$ of the pool.
We want t for which $\displaystyle \frac{1}{1}$ of the pool is full (the whole job).
$\displaystyle \frac{t}{8}+\frac{t}{24}=1\qquad$
... LCD=$24$
... multiply with $24$
$3t+t=24$
$4t=24$
$t=6$ hours
