Answer:
X=120
Step-by-step explanation:
It is an equalateral so all the angles in the middle are the same all being 60
The angle of a straight line is 180
So just minus 60 from 180 to get 120
180-60=120
20-13=7+11=18 is the answer
Don't forget to thank me
Imagine a corner of a cube.
It is made of three planes interesting at a single point
The image should help.
About 400 marbles. You simply set up a proportion and solve it as I did in the photo attached.