All nonzero numbers have an infinite amount of multiples.
6 : 7
There are 3 such numbers: 30, 60, 90.
1 and 1
50
50
1 and 2
The multiples of 5 that are also multiples of 6 are multiples of their LCM. The LCM of 5 and 6 is 30, so the question becomes which multiples of 30 are less than 100? The solution is the numbers: 30, 60, 90
45, 54
There are 2 such numbers: 35, 70
There are 2 such numbers: 44, 88.
To find prime numbers less than 100, the sieve of eratosthenes filters out 1 and all multiples of 2, 3, 5, and 7. All remaining numbers less than 100 are primes.