Answer:B
Explanation:
Given
Wavelength of light 
Screen distance 
First fringe is at a distance 
No of lines per mm is given by N
where d=slit width
From N-slits Experiment
Position of bright fringe is given by
Put the value of 
in eq. 1
Therefore 
for 
Your answer is 20
just take 1,200 divided by 60 [second] :)
The "walkie-talkie" is Don Hings' most well-known invention. The earliest versions of this device were designed as portable field radios for the bush pilots of Consolidated Mining and Smeltin , who had to fly their planes between remote sites in the far north of Canada.
