Your error is in solving your first equation: when r = 1 then 2(pi)r = 6.28 (to 2 d.p.), not 2.68.

You'll then find that 25.12 / 6.28 = 4, as expected.

## Number theory

Your error is in solving your first equation: when r = 1 then 2(pi)r = 6.28 (to 2 d.p.), not 2.68.

You'll then find that 25.12 / 6.28 = 4, as expected.