Let f:S → (0,1) given by f(x,y) = z with
z=0.A1B1A2B2… whenever x = 0.A1A2… and y=0.B1B2…
Then f is well defined and 1-1, and so |S| ≤ |(0,1)|
But we already know from 1st part of the exercise that |(0,1)|≤ |S|
Thus Schroder-Bernstein Theorem conclude that |S| = |(0,1)| or S ∽ (0,1)