The town of Malmo, Sweden has only two late-night food options: Pizza and Kebab. All sellers of late-night food have either a street permit or a permanent store permit. 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. If Malmo’s ratio of total street permits to total permanent store permits is exactly 7 to 3, what percentage of all late-night food sellers in Malmo serve pizza?
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.
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