Answer :

You can almost always translate a ratio into a fraction, because a fraction is a ratio.  Therefore, 10:20 become 10/20 (10 over 20), which we can then simplify by dividing both numerator and denominator by 10, which equals 1/2 (1 over 2), or 1:2.

I hope I helped!  If you need anymore information, please tell me! :D
Almost always the easiest way to simplify a ratio is to put it into fraction form.  In ths case it would be 10/20.  Since this is a proper fraction, you divide the denominator by the numerator (always the bigger number, in improper it's the numerator by the denominator..)  this would equal two, and you put it over one to make a fraction, so the answer is 1/2; 1:2; or 1 to two..