According to the results of a survey by Lake Enthusiast Magazine, 25%

The question doesn't really make sense, because a canoe is a boat. So if you own a canoe, ipso facto you own a boat. But if we assume canoes and boats are completely different things, then if the question asked "what percent of subscribers own a canoe but do not own a boat?" the answer would be 21%. But the question asks something different; it asks "what percent of subscribers who own a canoe do not own a boat". So our denominator is only those people who own a canoe, and not all of the magazine subscribers. And if you fill in a Venn diagram with 100 people in total, there are 30 who own a canoe, and 21 of them do not own a boat, so 21/30 = 70% is the answer. It can be hard to tell looking at a matrix what all of the numbers represent, so I find these questions much easier to answer with a Venn diagram.