Finding original concentration w%/w%
$$\frac{S +1}{W + S + 1} = 1/3$$
$$\frac{ S + 1}{W+S+2}= 3/10$$



$$S +1=\frac {W + S + 1}3$$
$$S + 1=(W+S+2) \cdot 3/10$$



$$3S +3=W + S + 1$$
$$10S + 10=(W+S+2) \cdot 3$$



$$W=2S +2$$
$$10S + 10=(2S+2+S+2) \cdot 3$$
$$10S + 10=9S+12$$
$$S=2$$
$$W=6$$



The initial concentration is 25 % (w/w).