Factors & Multiples :
If x divides y exactly, x is a factor of y and y is a multiple of x.
Co-primes :
Two numbers are said to be co-primes if their H.C.F. is 1.
H.C.F. and L.C.M. of Fractions :
H.C.F = H.C.F of Numerators L.C.M of Denominators L.C.M = L.C.M of Numerators H.C.F of Denominators
Note : Product of two numbers = H.C.F x L.C.M