How can i find the missing length for the triangles in each pair are similar

If these two triangles are mathematically similar, then we know angle A on the smaller triangle is the same as angle A on the bigger triangle.

That means it's safe to assume if you flipped the triangle around to make it so that C = W and B = V (as shown in the picture)

That means you can do 16 / 8 to find the scale factor, as the lengths are similar, so the scale factor is 2.

Apply the same logic to the 20cm length, and do 20 / 2 (the scale factor) to get 10cm for the smaller length.