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 ##2cdot 5## and 20 as ##2cdot 2cdot 5##.

Thus, the fraction ##frac{10}{20}## can be written as ##frac{2cdot 5}{2cdot 2cdot 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}##.