Hello student,
1)Electric field in the space between the plates towards right
E=

So work done W=Fs cos

=qEacos (

)
=

2) Initial number of nuclei N0 0
Final number of nuclei N N0-N
N=N0(1/2)nwhere n is number of half lives=(1/3) in this case
(N/N0)=1/21/3so (N0-N)/N=0.259
Thanks and Regards
Shaik Aasif
askIITians faculty