Figure given above illustrates the informations given in the question.
Using polar coordinate system we can calculate the width d of field region as
d = R sin60 - R sin 30 = R[ (√3/2) - (1/2)] .......(1)
we know in magnetic field region a charge of q Coulomb moves in a circular path and force acting on the charge is q×B×v,
where B is magnetic field strength and v is speed of charge.
hence we have
from (1) and (2), we have