General question about wheels and tires

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To make the story short I have 235/45/17 tires on 7" wide wheels. I went to relace a cracked tire at a used tire place. He shows me a used one that size and I agreed to buy it and have it installed. Well they show me my tires were badly unevenly worn and real low on one side. They then bring out two other identical tires. Continental sport contact 2 ZR17 with about 70% tread left and we made a deal and these were put in the front. The car drives great. I didn't realize it but they sold me a slightly wider tire than I went to replace. This is how it reads on the tire. 245/240/45/17ZR 91Y The tire does bulge out a little from the rim. Will this be OK and safe? I have 235/45/17 on the rears. I know the 245 is the tire width. Why are there two numbers 245/240? Which one is it? Thanks in advance for the input. From what I have read the tires will cup and not wear that well but I will replace all four later on.

Thanks
Mike