Hello,
1. Formula
circle's perimeter = 2 × π × r
you want the perimeter of half a circle :
half circle's perimeter = π × r
2. Application
diameter = 20cm
radius = 20/2 = 10cm
P = π × 10 ≈ 31,41cm
3. Conclusion
The perimeter of half a circle if the diameter is 20cm = 31,41cm.