<PRE><B>Question 855701</B>
x how much to add or use


{{{(30x+200*70)/(x+200)=50}}}


Your initial setup seems good.  Maybe you let the switch to decimal fractions confuse you?


{{{30x+200*70=50(x+200)}}}
{{{3x+200*7=5(x+200)}}}

{{{3x+1400=5x+1000}}}

{{{highlight(x=200)}}}, and in fact this works as correct when tried as a check in our original equation.


(Seemed strange to me also, ...until examining the AVERAGE of 30 and 70.  Their middle value IS 50.)</PRE>