Answer
(A) 12
Work Step by Step
From the problem, we know Luis raised 21 dollars, earning 1.75 dollars for every mile (x) he ran:
$1.75\times x=21$
We can now isolate $x$ by dividing both sides of the equation by $1.75$:
$(1.75\times x)/1.75=21/1.75$
$x=12$
