To the managers of this forum

It might help students more if instead of lambasting problem creators or the
students for miscopying problem or for not understanding what we think they
ought to understand, that we make up sensible problems similar as possible to
what they posted.  So I decided to do that instead.

I won't mix grams and kilograms, since it would be unusual and awkward to
measure weights of candy bars in kilograms. Also I'll change the 9% to 20% so
the answer will come out prettier. Here goes:

Suppose previously, a candy bar had sold for price p cents and weighed w grams.
 
Therefore the OLD price per gram was
 
 or 

Now the weight has been changed from w grams to 100%-20%=80% of w grams or 0.80w
grams.

Therefore the NEW price per gram is

  or  

Now we first compare the NEW price per gram to the OLD price per gram by
dividing the NEW by the OLD:



Invert and multiply:







Since the NEW price is 125% of the OLD price, it means the
new price has been increased 25% (over 100% of the OLD price).

So even though we cannot know the OLD or NEW price or the OLD
or NEW price per gram, we still know that the OLD price per 
gram, whatever it was, was increased by 25%.

Edwin