hmm, one way is this
there's 500 times 0.10 or 50 grams of salt in the water
if we want to make 50 grams=4% of everything, how much water do we need to add
50=4% of all
50=0.04 of all
divide both sides by 0.04
1250=all
so
500+what=1250
minus 500 both sides
what=750
750 grams of water
