Consider numbers of the form , where is a positive integer. We shall call such a number ‘grime’ if it cannot be expressed as the product of two smaller numbers, possibly equal, both of which are of the form , where is a positive integer.
How many ‘grime numbers’ are there in the sequence 11, 21, 31, 41, …, 981, 991?
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0
8
87
92
99
Select an option, then submit.
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