Since we are given the radius of the circle, we can immediately calculate the circumference of this circle using the equation 2πr (2π(4) = 8π). Now that we know the circumference (C=8π), you can give the circumference a common denominator with arc AB (5). We can multiply 8π by 5/5 to get 40π/5. We can now see that 4π is 1/10 (or 0.1) of 40π/5 (or 8π). The answer is 1/10.