A correspondent asked me whether it is ok to say 'these kind of questions'. The usual advice given regarding sentences such as these is that they are ungrammatical, and it would be better to say 'this kind of question', making everything singular, or 'these kinds of questions', making everything plural.
However, the 'these kind of' structure is very common in speech, and there are as many examples of 'these kind of' as there are of 'these kinds of' in the British National Corpus. The OED tells us that the phrases 'all kind of'' and 'no kind of' were once treated more or less as adjectives, and since adjectives don't inflect in English, the 'kind' in these expressions never varied, even when followed by a plural noun. Shakespeare, for instance, writes in King Lear "These kind of knaves I know". And Jane Austen wrote in Sense and Sensibility "I know so little of these kind of forms ...". 'Kind' is related to the word 'kin', and since 'kin' has no plural, or, rather, it is already a sort of collective noun, so 'kind' was considered plural in meaning too. I've found one citation containing 'kinds' in the plural in Johnson's Dictionary; it's "Thus far we have endeavoured in part to open of what nature and force laws are, according to their kinds" from Richard Hooker, who was writing in the late 16th century, but it wasn't used much before this, as far as I can ascertain.
My advice is to stick to the 'these kinds of questions' structure, but don't worry too much if you say 'these kind of' -- you're in good company!