Question 1128194
Kelly earns $36 for 3 hours of work. At that rate, how long will she have to
work to earn $720?
<pre>
$36 IS TO 3 hours AS $720 IS TO X hours.

"IS TO" means "OVER".  "AS" means "EQUALS"

$36 OVER 3 hours EQUALS $720 OVER X hours.

{{{matrix(1,3,

"$36"/3, ""="", "$720"/X)}}}

CROSS MULTIPLY, that is, 

"UPPER LEFT times LOWER RIGHT equals UPPER RIGHT times LOWER LEFT":

{{{matrix(1,3,"$36"X, ""="", "$720"*3)}}}

{{{matrix(1,3,"$36"X, ""="", "$2160")}}}

Divide both sides by $36

{{{matrix(1,3,("$36"X)/"$36", ""="", "$2160"/"$36")}}} 

{{{matrix(1,3,X, ""="", 60)}}} 

Answer: 60 hours

Edwin</pre>