Add the ratio terms to get the whole. Use this as the denominator. 1 : 2 => 1 + 2 = 3.
Convert the ratio into fractions. Each ratio term becomes a numerator in a fraction. 1 : 2 => 1/3, 2/3.
Therefore, in the part-to-part ratio 1 : 2, 1 is 1/3 of the whole and 2 is 2/3 of the whole.