Given, Soumya was born on 30 January 1984, and her cousin Sundeep was born on the 40th day after the day of her birth.
To find the day of Sundeep birth, find 40th day from 30 Jan; 1984.
Month: Jan Feb Mar
Days: 1(31 - 30) + 29 (1984 is leap year) + 10 = 40 days
So, 40 days after 30 Jan;1984 is 10th March;1984.
Hence, Sundeep was born on 10th March;1984.