A durian seller sells half of his durians plus half a durian to his 1st customer. he then sells half of the remainder plus half of a durian to his 2nd customer and half of the remainder plus half a durian to his 3rd customer. he repeats the same process for the next customer and so on. after he has served the 7th customer, he finds that he has sold all his durians. how many durians had he originally?

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