In quadrantal bearing system, back bearing of a line may be obtained from its forward bearing, by

adding 180°, if the given bearing is less than 180°
subtracting 180°, if the given bearing, is more than 180°
changing the cardinal points, i.e. substituting N for S and E for W and vice-versa
none of these.

Correct Answer: changing the cardinal points, i.e. substituting N for S and E for W and vice-versa

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