I think this is a poor-quality question and I don't agree with the explanation. Some examples of SG primes are 23, 47, 59 etc.
Whereas in explanation and calculation, unit digit of SG prime are considered as 1, 3 and 9 which is wrong. Unit digit of SG prime can only be 3, 7 and 9 and therefore, product of 3,7 and 9= 189 must be the answer.
Whereas in explanation and calculation, unit digit of SG prime are considered as 1, 3 and 9 which is wrong. Unit digit of SG prime can only be 3, 7 and 9 and therefore, product of 3,7 and 9= 189 must be the answer.
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