The question which arose my confusion:
Two resistors with resistance $R_1 = 100\pm 3 \ \Omega$, and $R_2 = 200\pm 4 \ \Omega$ are connected in (a) series, (b) in parallel. Find the equivalent resistance of both combinations.
In the end, my answer came out to be (a) $300\pm 7 \ \Omega$ and (b) $66.7\pm 2 \ \Omega$. But according to my book, my response for (b) is incorrect.
It said that the equivalent resistance should have an error of $\pm 1.8$ not my rounded off answer $\pm 2$. And it (book) explicitly states that the error in (b) should be "expressed as $\pm 1.8$ to keep in conformity with the rules of significant figures." I don't understand how this makes any sense (especially how it conforms to the rules of significant figures). Help anyone?