First add the ratio 1:3:5 together to give you the total number:
1 + 3 + 5 + = 9
Therefore you divide 72 by 9 to give you one part
72 divide 9 = 8
Now you times each part of the ratio by 8:
1 x 8 = 8
3 x 8 = 24
5 x 8 = 40
Therefore the answer would be written like this:
8:24:40
Hope this helps