Mam please tell the solution multiple choice question 3th

dear student

watches sold in 1 day=w
watches sold in 7 days (1 week) = 7 * w

watches sold in n weeks = 7 * w * n
total watches =300

suppose watches left =x

so
x= 30 - (7 * w * n)
  =30-7wn

so the answer is option ( d)

regards