Question 765281
This is what you want to understand about a dilution:


{amount of salt initially}/{amount of final solution}={concentration of final solution}


Amount of starting salt:  500*0.10 grams

Amount of water to add for dilution:  let this be w

Amount of resulting solution of salt:  500+w


EQUATION:
{{{highlight((500*(0.10))/(500+w)=0.04)}}}
That uses decimal fractions and not percents.
SOLVE FOR w.