physics help very confusing error question?

[princess]

You measure the moment of inertia of a wheel by measuring its angular acceleration a = 0.404 rad/s2, when applying an external torque caused by a hanging weight of mass m = 200 g as shown above. The angular acceleration is decreased by a frictional torque, accounted for by afric = - 0.165rad/s2 in the expression for the momemt of inertia I of the wheel, I = mr(g-ra) /(|afric|+a), where r is the radius of the cylinder attached to the wheel, and g is the acceleration of gravity, which you treat as error free. You neglect the error of the accelerations a and afric, and the error of the mass m. You find out that the term (a r) in the numerator is small compared to the term g, and thus neglect its error too. What is the absolute error of I, if the 2.5 cm radius r is known with a 1.38% accuracy. (One factor contains the quantities assumed to be error free, after taking into account that the term (a r) can be neglected for the purpose of error calculation.