Simplifying a fraction #\frac{p}{q}# means to rewrite it as an equivalent function #frac{m}{n}# such that #\frac{p}{q}=\frac{m}{n}#, but #m# and #n# have no common divisors.
In your case, the fraction #\frac{10}{20}# is not simplified, because both numbers are divided by 2 and 5.
Considering the generale case, if you want to simplify a fraction, you can write both numerator and denominator as the product of their prime factors, and then simplify all common primes.
In your case, one can write 10 as #2\cdot 5# and 20 as #2\cdot 2\cdot 5#.
Thus, the fraction #\frac{10}{20}# can be written as #\frac{2\cdot 5}{2\cdot 2\cdot 5}#. We can simplify one 2 and one 5 from both numerator and denominator, and so we conclude that #\frac{10}{20}# is simplified into #\frac{1}{2}#.
