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If you say a:b=1:2 this means a/b=1/2 or a=1/2 of b. On the other hand suppose we were asked to divide say £12 in the ratio 1:2 then one part=1/3 of 12 =£4 and the other part is 2/3 of 12=£8 Notice the answers are in the ratio 1:2 as asked.
As a Fraction:
the fraction of girls is 10/15 = 2/3,
the fraction of boys is 5/15=1/3
As a ratio:
the ratio of boys to girls is 1:2,
the are half as many boys as girls,
there are twice as many girls as boys.
the confuson arrises as people often mix fractions into ratios to make the number easier to handle