If you want a group under multiplication, the easy options are: (1) Integers modulo a prime number, not counting 0 mod p. (2) Integers modulo anything (call it n), not counting ones that have a common factor with n. (3) Rational numbers.
In this case you've got 1..100, and 101 is prime. So the numbers on (the front of) the sweater, mod 101, form a group under multiplication. But of course in that group you lose all the prime-number structure shown on the sweater -- you can't really talk about, say, "multiples of 3 (mod 101)" because 101 isn't a multiple of 3.
In this case you've got 1..100, and 101 is prime. So the numbers on (the front of) the sweater, mod 101, form a group under multiplication. But of course in that group you lose all the prime-number structure shown on the sweater -- you can't really talk about, say, "multiples of 3 (mod 101)" because 101 isn't a multiple of 3.