It is impossible to do this with less than 6 guesses since we are guessing within 1-100. The maximum space we can guess in with less than 6 guesses is 81 = (3^(5-1))$81 = 3^{5-1}$. Less than 5 guesses would be 27$27$, and so on. However, you could guess in a space of up to (3^(6-1)) = 243$3^{6-1} = 243$ with 6 guesses.