One scientist kept 64 grams of a radioactive material in a container. after how many half lives will 1 gram of radioactive material be left in the container?

For this case we have an equation of the form:
 y = A * (b) ^ t
 Where,
 A: initial amount
 b: decrease rate
 t: periods of time
 Substituting values:
 y = 64 * (0.5) ^ t
 For y = 1 we have:
 1 = 64 * (0.5) ^ t
 Clearing t:
 (0.5) ^ t = 1/64
 log0.5 ((0.5) ^ t)) = log0.5 (1/64)
 t = log0.5 (1/64)
 t = 6
 
 it will be 1 gram of radioactive material left in the container after:
 6 half lives

These graph is cross the yaxis and x axis two ways.