13)
we know their product is 2400, so that means whatever those two numbers are, they're factors, so we can do a quick prime factoring of 2400 and shuffle the factors a bit to get them.
2400 = 2*2*2*2*2*3*5*5
120 = 2*2*2*3*5
20 = 2*2*5
the LCM or LCD of 120 and 20 is well, you guessed it, 120, hmmm what's their GCF or HCF? well, that'd be 20, 20 is a factor of both and is the GCF.
14)
we know their LCD or LCM is 300, so we can do a prime factoring of 300, and use those factors, keeping in mind that those two numbers are not factors and therefore don't have to produce a product of 300, so it can higher.
300 = 2*2*3*5*5
20 = 2*2*5
300 = 2*2*3*5*5
the LCD or LCM of 20 and 300 is, well, yeap is 300, and their GCF or HCF is 20, 20 is factor of both and is the largest one can tease out of there.