In a first order reaction, t 1/2 does not depend on concentration of reactant.
t 1/2 (or half life) implies time in which half the reactant is consumed.
t 3/4 implies time in which 3/4 of the reactant is consumed.
After 1 half life, 1/2 of the reactant will be consumed.
After 2 half lives, 1/2 of 1/2 i.e. 3/4 of the reactant will be consumed.
so, time for t 3/4 is 2 x 32 = 64 minutes.