#### Answer

$8-x=15\div5$
$x=5$

#### Work Step by Step

"Decreased by" indicates subtraction. "Quotient" indicates division.
$8-x=15\div5$ $\longrightarrow$divide
$8-x=3$ $\longrightarrow$addition property
$8-x-8=3-8$ $\longrightarrow$simplify
$-x=-5$ $\longrightarrow$multipliccation property
$-x(-1)=(-5)(-1)$ $\longrightarrow$simplify
$x=5$