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To solve this problem we must go step by step.

Firstly, we know the number of garlic cloves is not a multiple of 3,4,5 and 6 as Ben could not make any equal rows out of them.
Secondly, we know that the number of cloves is less that 100.

So, using this information, we can deduce that the only possible numbers left are the prime numbers between 1 and 100 and some other lonely numbers including 49.

We also know that it cannot be an even number because 4 is a multiple of 2 and Ben tries to make it up with 4 equal lines.

Finally, we know that it is made of equal rows meaning it must be a square number.

Therefore, the only numbers it can be is 49 and 81.