Differential privacy of "randomized responses"
I found the answer in this book  https://www.cis.upenn.edu/~aaroth/Papers/privacybook.pdf at page 30


  Fix a respondent. A case analysis shows that 
  $$Pr[Response = Yes|Truth = Yes] = 3/4$$ 
  Specifically, when the truth is “Yes” the
  outcome will be “Yes” if the first coin comes up tails (probability
  1/2) or the first and second come up heads (probability 1/4),
  while $$Pr[Response = Yes|Truth = No] = 1/4$$ (first comes up heads and
  second comes up tails; probability 1/4). Applying similar reasoning to
  the case of a “No” answer, we obtain:
  $$
\frac{Pr[Response = Yes|Truth = Yes]}{Pr[Response = Yes|Truth = No]}=
\frac{3/4}{1/4}= \frac{Pr[Response = No|Truth = No]}{ Pr[Response = No|Truth = Yes]} = 3$$