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Hint:
spoiler
The size of a set is the number of possible values that an element can take.
I think this question is very poorly worded
I read the digits as being part of a product operator
Sure, but what is d_\infty? What is the set \infty? What does the question mean by “every real number can be represented by”? There’s nothing distinguishing d_1d_2… with d_1 times d_2 since we normally denote the product by writing the variables next to each other