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by aleph_minus_one 742 days ago
> Also, wouldn't the tolerance be doubled when you add them in series? Or does it still average out to +/- 5%?

Neither.

Let R_{1, ideal}, R_{2, ideal} be the "ideal" resistances; both with the same tolerance t (in your example t = 0.05).

This means that the real resistances R_{1, real}, R_{2, real} satisfy

(1-t) R_{1, ideal} ≤ R_{1, real} ≤ (1+t) R_{1, ideal}

(1-t) R_{2, ideal} ≤ R_{2, real} ≤ (1+t) R_{2, ideal}

Adding these inequalities yields

(1-t) (R_{1, ideal} + R_{2, ideal}) ≤ R_{1, real} + R_{2, real} ≤ (1+t) (R_{1, ideal} + R_{2, ideal})

So connecting two resistors with identical tolerance in series simply keeps the tolerance identical.