Consider the product of intervals A and B
- A: center Ca > 0 and error percentage Pa
- B: center Cb > 0 and error percentage Pb
where Pa and Pb are small

A = (Ca(1 - Pa), Ca(1 + Pa))
B = (Cb(1 - Pb), Cb(1 + Pb))

A*B = (CaCb(1 - Pa)(1 - Pb), CaCb(1 + Pa)(1 + Pb))
    = (CaCb(1 - Pa - Pb + PaPb), CaCb(1 + Pa + Pb + PaPb))
    ~ (CaCb(1 - (Pa + Pb)), CaCb(1 + (Pa + Pb)))
Has center CaCb and error percentage ~ Pa + Pb