What's the highest common multiple of 16 and 24

The highest common multiple???  That would be infinity...

Highest common factor?  That would be the product of shared primes of the numbers prime factorization...

16=2*2*2*2, 24=2*2*2*3 so the product of shared primes is 2^3=8

So 8 would be the greatest common factor

Least common multiple?

That would be the product of the highest occurring primes in their prime factorization...

16=2*2*2*2, 24=2*2*2*3 so the product of highest occurring primes is:

2*2*2*2*3=48

So 48 would be the least common multiple

The "highest common multiple" would be 48n where n would approach infinity so it would be +oo