#### Answer

$x=11$

#### Work Step by Step

Let x = the unknown number
Then
"the sum of 5 and a number" means $5+x$
"3 times the sum" means $3(5+x)$
Thus, the equation that represents the situation is:
$3(5+x)=48$
Use the distributive property to simplify.
$(3\times5)+3x=48$
$15+3x=48$
Use the addition property of equality to subtract 15 from both sides of the equation.
$15+3x-15=48-15$
$3x=33$
Use the multiplication property of equality to divide both sides of the equation by 3.
$3x\div3=33\div3$
$x=11$