I am currently trying to study for my IGCSE maths paper... I am stuck on this question ... can anybody help??
If 3/5 of people in a theatre buy a snack during interval, and of those who buy a snack 5/7 buy ice cream, what fraction of the people in the theatre buy ice cream??
Note for the future: generally, people on Math.SE don't like it when you ask 'homework questions' if you don't show your attempts. It also helps to provide some context, though the degree to which this is necessary varies from question to question. In this case, you mentioned that it was from an IGCSE maths paper, which to me is enough context for people to gauge what level of mathematics you are working at, meaning that people can tailor their responses.
Now that is out of the way, here is a hint to your question:
What we are looking to do is translate this wordy problem into an equation that we can manipulate. Pay attention to the details: '$3/5$ of people in a theatre buy a snack during an interval'. In other words, for every $5$ people, $3$ of them buy a snack. Now, what is $5/7$ of $\frac{3}{5}$? Pay attention to which operator ($+,-,\div,\times)$ you are using when you calculate the answer.