Hi LeenaSai,
The prompt gives us a 'recipe' for what each cookie needs to be: 1/5 of it is chocolate and the 'rest' is dough. For example, if a cookie is 1 ounce chocolate, then we would need 4 ounces of dough - since that would be a 5-ounce cookie and 1/5 of that cookie would be chocolate.
Since the question asks how much chocolate is LEFT OVER, we clearly have more chocolate than we need. The question is 'how much?' With the above example, we can see a 'ratio' between the amount of chocolate and the amount of dough per cookie - it's 1:4. With 36 ounces of dough, we can 'group' the dough into 4-ounce groups - meaning that we would have 9 cookies with 4 ounces of dough each; we would then add 1 ounce of chocolate to each of those cookies (which would turn them into 5-ounce cookies). That would require 9(1) = 9 ounces of chocolate and 15 - 9 = 6 ounces of chocolate would be left over.
The calculation that you did calculated the total weight of the cookies IF ALL 15 ounces of chocolate were used AND we kept the same 1:4 ratio AND we had enough cookie dough to make all of those cookies. That's not what the question told us though (or asked us to solve).
GMAT assassins aren't born, they're made,
Rich
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