The equation for the circumference of a circle is:
Circumference = pi × diameter
(or, C = πd )
And so for your equation C = 55.3,
So,
C = πd
55.3 = πd
So now we can divide both sides of the equation by π to find out what 'd' equals:
55.3/π = (πd)/π
55.3/π = d
And so since the diameter of a circle is the same as 2 multiplied by the radius, this means that:
d = 2r
So you can substitute this fact into your equation:
55.3/π = d
55.3/π = 2r
2r = 55.3/π
(2r)/2 = (55.3/π)/2
r = 55.3/(2π)
r = 8.80126835...
And this number rounded to the nearest tenth is:
r = 8.8