While you need a basic understanding of percentages to get to the numbers of pupils, to solve the problem you need to be able to figure out the logic of it. I don't think this is a typical "percentages" question, but might be a typical logic problem requiring clear analytical thinking and being able to translate the question into a logical approach.
The "trick" to this one is to see that there are three groups of pupils: Those doing 2 languages (none do more, we are told) =250 Those doing 1 = ? Those doing none = the question being asked. The question asks for the number doing none, but to get there you have to work out how many do only 1 first, which is not directly asked for nor given.
An approach to getting your head round problems like this is to realise that what is being counted is not really pupils, but the intersection of a pupil and a language. A good mental proxy for this is the text book the pupil has, so we can say: There are 300 French text books, 100 Latin, 200 Spanish = total of 600 text books shared between the pupils studying one or more languages. 250 pupils learn two languages so they have two books each. This is 500 of the total number of books, leaving 100 for the pupils studying only 1 250 + 100 = 350, leaving 150 studying none.
Hope this helps.
