Let the radius of yellow semicircle be r,

join the two centers of yellow and blue semicircles and this line intersets them at the point of tangency

hence we get a right triangle with hypotnuese = 18 + r

base =18

perpendicular = 36-r

```
applying pythagoras theorem we get (18+r)^2 = 18^2 + (36-r)^2
we get r= 12 cms
```