There are 3 parts to the ratio so...
First we divide the total number of objects we are sharing (in this case 15 biscuits) by the number of parts in the ratio (3)
15 / 3 = 5
Then we know that each part of the ratio is worth 5 biscuits. So we times each part of the ratio by how many biscuits are in one part...
1 x 5 = 5
2 x 5 = 10
So when the biscuits are shared into the ratio 1:2, the number of biscuits in each are 5:10
Hope this helps :)