The town of Malmo, Sweden has only two late-night food

Excellent explanation by PlaPon.
I want to add my 10 cents:
1. In these type of questions we have to assume that Total number is 100. we really do not need to fight with %
2. This particular question asks us to make an assumption second time when it says street/permanent=7/3. It is clear for us that it is 70/30.
3. Now it is an easy 550 level question!

It seems to me that the problem statement is somewhat unclear. It is still possible that some of the late-night street vendors serve both pizza and kebab.

Thus, the answer is between 16% and 76%.

In my mind, the grain of doubt is minuscule, if at all. It is the way the question is written.
Sure, in sets, 60% vendors serve kebabs implies that the other 40% do not serve kebabs. It doesn't imply that only 40% serve pizzas. It is certainly possible that some or all of those 60% serve pizzas too. So the % that serves pizza could be higher. But two things weigh in favor of the intended interpretation.

1. 'that serve kebab' qualifies 'street vendors'. We will say that '60% are street vendors that serve kebab' only if we mean '60% are street vendors that serve only kebab and no other street vendor serves kebab'. If some of those 60% serve pizza too, we must say '60% are street vendors that serve at least kebab'. Of course, to remove all doubts, a better idea would have been to write it clearly.

2. It is a PS question so the intended meaning is quite obvious.

"60% of all the late-night food sellers in Malmo are street vendors that serve Kebab; 20% of all the late-night food sellers who have a permanent store serve Pizza" is ambiguous. It means the total number of street vendors is 60 and they all serve kebab. It is contradictory to the ratio of 7:3.

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