The grammar in your answer is not too bad (other than omitting a verb, some punctuation issues, and such), but you’ve misunderstood the question.
The question doesn’t say that 90% of Spanish advertisements are about food. It does not say that 90% of Spanish food advertisements are aimed at children. It is *only* talking about Spanish advertisements that are about food, and that are also aimed at children. Ninety percent of *those* advertisements are for unhealthy food, according to the question.