the time taken for the completion of 3/4th of a reaction to complete is?

t /2 = .693 / k.     for half life of the reaction.

t = 2.303 /k      log  a / (a-x);  when x = 3 /4 a, t = t/2.

t₂ = 2.303 /k  log    a / a - 3/4a

or 2.303 x 2 /k   log  2

2x 2.303 x .30 /k 

2x.693 /k   =2xt₁

 

time taken for 3  / 4 of a reaction is twice of the half period in a first order reaction.