Introduction
These types of "English into Algebrese" questions seem to cause students a lot of problems.
In essence, the approach to use is:
INITIAL STATE + ADDITION = FINAL STATE
SO, lets look at an example.
EXAMPLE
Q: A vat contains 90kg of powdered milk and 10kg of sugar. How much more sugar must be added to the mixture to make a 25% sugar mix?
A: Consider the sugar, as that is what the question is about.
Initially: the total amount in the mixture is (10+90) --> 100kg.
We therefore get that (10/100) is the fraction of sugar, initially.
Now add some sugar, Let this be x. This will increase the amount of sugar and also the total amount of the mixture, giving:
Finally: (10+x)/(100+x) is the new fraction of sugar.
We are told that the final fraction of sugar is 0.25 (--> the 25%: ie 25 out of 100)
So, we have

--> x = 20kg
and now
CHECK that it makes sense:
final mix is

, which becomes

--> 1/4
--> 25% so therefore correct.
EXAMPLE
Q: How many kilogram of fine glucose must be added to 25 kilograms of a 12% sweetened juice concentrate to prepare instead a light syrup 20% sweet?
A: Consider the glucose, as that is what the question is about.
Initially: there is 12% glucose in the mixture. This is 12% of 25kg. In other words, (12/100)*25 --> 3kg of glucose.
So, we have 3/25 as the initial fraction of glucose --> the 12%.
We then add some more. Let this be xkg.
Finally: we end up with

as the fraction of glucose. We are told that this needs to be 20% ie 0.20. So we have

--> x = 2/0.80
--> x = 2.5kg
Now, do the
CHECK
we had

, which becomes

--> 0.20 which is 20%.