The ratio of two numbers is 5 : 7 and their HCF is 3. Their LCM is:

Ratio of two numbers = 5 : 7 

H.CF = 3 

So,

The numbers = 5 × 3 = 15 and second number = 7 × 3 = 21

Now, we know that 

Product of two numbers = H.C.F × L.C.M 

⇒  15 × 21 = 3 × L.C.M 

L.C.M  = 105