Question 982018
if the variable {{{Z}}} is directly proportional to {{{X}}}and inversely proportional to {{{Y}}}, then  {{{highlight(Z=kX/Y)}}}

When {{{X =12}}} and {{{Y=4}}},{{{ Z =57}}}:

Your first sentence:  {{{Z=kX/Y}}}


solve for {{{k}}}:

{{{k=ZY/X}}}


Your second sentence indicates:
{{{k=57*4/12}}}

{{{k=57(1/3)}}}

{{{k=19}}}


since the formula for {{{Z}}} is:

{{{highlight(Z=19Y/X)}}}  

 then the value of{{{ Z}}}, when {{{X = 18}}} and {{{Y = 10}}}, will be:

{{{Z=19X/Y}}}
{{{Z=19*18/10}}}
 {{{Z=342/10}}}
{{{Z=34.2}}}