#### Answer

$\frac{3}{10}$

#### Work Step by Step

1. Write numerator and denominator as product of primes:
$\frac{12}{40}$ = $\frac{2 * 2 * 3}{2 * 2 * 2 * 5}$
2. Separate common factors by writing as fraction equal to 1:
$\frac{3}{2 * 5}$ * $\frac{4}{4}$ = $\frac{3}{2 * 5}$ = $\frac{3}{10}$