mathematics... help please?

Question

Scientists expect that in future space stations, artificial gravity will be created by rotating all or part of the space station. The formula N = 1 / (2*pie) * sqrt(a/r)

gives the number of rotations N required per second to maintain an acceleration of gravity of a meters per second squared on a satellite with a radius of r meters. The acceleration of gravity of Earth is 9.8 m/s2. How many rotations per minute will produce an artificial gravity that is equal to half the acceleration of gravity on Earth in a space station with a 15 meter radius?

Answer 1

sqrt(4.9/15)/(2*pi) = 0.09096

Thus Per minute = 5.5 rotations.