Trading a/c.....query

There is a question related to trading a/c said it that

(a) Stock of goods at 10% lower than cost as on 31 mar 2014 .......108000

Answer.......... 120000

My query is in above question said it that ..........stock is less than 10% then why not solve this question in that manner

108000*10%=10800..........now, 108000+10800= 118800  ans.

But the actual ans is 120000 by dividing 108000/90%.....why using that method for solving the question.

Plz clear my doubts........