First factor the each term in the expression.
\100x3 = 2 · 2 · 5 · 5 · x · x · x
\75y3 = 3 · 5 · 5 · y · y · y
\The GCF is the product of the common prime factors in each term.
\GCF = 5 · 5 = 25
The greatest common factor of 100x3 and 75y3 is 25.