Ok. In general is the absolute error either +ve or -ve? Is an error bound always positive? Wikipedia seems to muddle these things up.
It would be positive,
however that's because the modulus is taken,
which doesn't mean we get the same error here if the radius is lower by 0.25%
or if it's higher by 0.25% from it's nominal value.
To calculate the absolute error you've taken the minimum value of r so you must also take the minimum value of pi which is 3.1415. This will get the answer in the book. Part b is done similarly for pi=3.14159.