A girl, A, told her friend about the colour and length of a snake she saw on…

Concept

A statement of the form "If X is false or Y is false, then Z" is automatically true — with no constraint on Z at all — in exactly one situation: when X and Y are BOTH true (then "X is false or Y is false" is itself false, so the implication holds vacuously). In every OTHER situation (X and Y not both true, i.e. at least one of them false), the implication forces Z to actually hold. This gives the standard assume-and-eliminate technique for compound-conditional puzzles: assume Z is false, and check whether the one exception case (X true and Y true) can hold together across every clue; if the exception cases the different clues demand contradict one another, the assumption is impossible and Z must be true.

Application

Assume, for contradiction, that the snake's length is not 55 cm. 1. Then clue 1 forces the exception case: colour = green AND length = 35 cm. 2. Then clue 2 forces its own exception case: colour = black AND length = 45 cm. 3. Then clue 3 forces its own exception case: colour = black AND length = 35 cm. These three forced outcomes cannot all hold together — they disagree on colour (green vs. black) and on length (35 vs. 45). So the assumption that length is not 55 cm is impossible, and the snake's length must be 55 cm.

Cross-check

Cross-checking: once length = 55 cm, the length is automatically different from both 35 and 45, so the 'or' part of every clue is already true regardless of colour — all three clues hold no matter which colour is paired with 55. Restricting to the pairs actually offered as choices, only one length-55 pair drawn from the stated colour set {brown, black, green} appears among them, so that is the pairing the clues single out.