First factor the numerator and denominator.
\17 = 1 · 17 \ \
\20 = 1 · 2 · 2 · 5
\The GCF is the product of the common prime factors in each term.
\GCF = 1
\
Since GCF of the numerator and the denominator is 1, the fraction already in its simplest form.
\\
Simplified.