A 3-gallon mixture contains one part S and 2 parts R.?

Question

A 3-gallon mixture contains one part S and two parts R. In order to change it to a mixture containing 25% S, how much R should be added.

Answer 1

1 part R

and it will be 4 gallons

Answer 2

1 gallon of R.

The 3-gallon mixture contains one part S and 2 parts R, so there is 1 gallon S and 2 gallons R. 25% S means 1/4 S, or 1 part S and 3 parts R, so we add 1 gallon of R.

^_^

Answer 3

1 part s and one part r means 1/3=s and 2/3=r
hence to make s 25% we have to make it 1/4 i.e. we have to add one more part of r that is r=3/4
thus we get 1/4 part of s which is 25%