Doubt by Raghav Singhal

can you explain question 41 of neetprep question bank a bit ? In the solution an expression is given y=7lamdaD/d , i am not able to figure out how

also the significance of calculating n1/n2 

https://www.neetprep.com/subject/U3ViamVjdDo1NQ==/topic/VG9waWM6NzAw/newquestions/40

Answers

Answer by Iqbal Hossain Sk

same problem , Mera bhi samajh mein nahi aaya