Question 804056
Let y = the amount of salt to add to change to 25%.


Amount of salt just as salt, {{{60*0.20+y}}}.
Amount of salt solution result,  {{{60+y}}}.


If 25% salt result is wanted, then {{{(60*0.20+y)/(60+y)=0.25}}}


Steps:
{{{12+y=0.25(60+y)}}}
{{{12+y=15+y/4}}}
{{{48+4y=60+y}}}
{{{48+3y=60}}}
{{{3y=60-48=12}}}
ANSWER:  {{{y=4}}} kg.



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You can set-up the solution for values of resulting salt solution higher than 25%, but beyond 25% salt solution would not be realistic.  Certainly anything as high as 40% will NOT happen, although you CAN set the initial equation for it and do the algebra/arithmetic steps similarly as shown.