Question 907018
Only a guess that the starting 30 is as fluid ounces and the salt is percent mass per volume.  The nurse would add some v fluid ounces of water:



{{{30*10/(v+30)=4}}}
{{{300=4(v+30)}}}
{{{4v+120=300}}}
{{{4v=180}}}
{{{v=(2*5*2*9)/(4)}}}
{{{highlight(v=45)}}} fluid ounces of water
(This checks as correct.)



CHECK ANSWER:
{{{30*10/(45+30)}}}
{{{300/75}}}
simplify
{{{25*12/(25*3)}}}
{{{12/3}}}
{{{highlight(4)}}}
Check is perfect.