Let the number of Sociology books =  y; the number of History books = x
⇒  x + y = 475   (2)
also since the number of history books sold was 83 less than sociology, 
then x = y - 83  (1)   
BY substituting (1) into (2)
⇒  (y - 83) + y = 475
                  2y = 475 + 83
                    y = (558 ÷ 2)
                    y = 279
⇒ x  =  279 - 83
     x  =  196
∴ Number of History books is 196 & number of Sociology books is 279.