You would find the radius by halving the diameter. Then you use the equation

and you get about 28.2 meters.
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Answer with explanation:
The equation of two planes are
1.→y +2 z=10
2.→x+y=11
To find the point of Intersection of two planes
let,→ y=t
→2 z=10 - t
10 -2 z=t
→x=11 -t
11 -x=t
So,Writing equation of line which is intersection of two planes
→ 11 -x = y= 10-2 z=t

---------------------------------(1)
⇒Putting , x=0, y=0 and z=0 in the above equation ,to find the point Closest to Origin which will be
=(11, 0, 10)