Question 159894
Let's call the number X.
{{{(30/100)X+(20/100)(80)=(28/100)(X+80)}}}
{{{30X+1600=28(X+80)}}}
{{{30X+1600=28X+2240}}}
{{{2X=640}}}
{{{X=320}}}
320 kg of 30% solution is needed.