#### Answer

Saturn has 18 moons.
Uranus has 21 moons.
Neptune has 8 moons.

#### Work Step by Step

Let $S$ be the number of moons on Saturn.
Let $U$ be the number of moons on Uranus.
Let $N$ be the number of moons on Neptune.
$U=N+13$
$S=2+2N$
$S+U+N=47$
$2+2N+N+13+N=47$
$15+4N=47$
$4N=32$
$N=8$
$U=N+13$
$U=8+13$
$U=21$
$S=2+2N$
$S=2+2(8)$
$S=18$