In a certain code language, ‘LIGHT’ is coded as ‘25709’ and ‘SIGHT’ is coded as ‘97065’. What is the code for ‘S’ in that language?
- (a)2
- (b)7
- (c)6
- (d)0
Answer
Why
Correct — C. Rule: the digits are jumbled, so compare the two words as sets rather than letter by letter.
LIGHT and SIGHT share I, G, H and T. The odd letter out is L in the first word and S in the second.
The codes 25709 and 97065 share the digits 9, 7, 0 and 5. The odd digit out is 2 in the first code and 6 in the second.
The four shared letters must take the four shared digits, which leaves L = 2 and S = 6 → option (c).
Why the others are wrong
- (a)2 — 2 is the digit that appears in 25709 and not in 97065, so it belongs to L — the letter that is in LIGHT and not in SIGHT.
- (b)7 — 7 appears in both codes, so it must stand for one of I, G, H or T, the letters the two words share. S is in SIGHT alone.
- (d)0 — 0 also appears in both codes and therefore belongs to a shared letter. Any digit common to the two codes is ruled out for S immediately.
Concept
When a code has the same length as its word but the digits are not in the word's order, matching by position fails and set logic takes over.
Two words that overlap in most of their letters let you isolate what is left. Whatever letter is unique to one word must be coded by whatever digit is unique to that word's code, because everything else has a partner on the other side.
You never learn the codes of I, G, H and T here, and you do not need them. The question asks for S alone, and the set difference delivers it in one line.
LIGHT and SIGHT differ in exactly one letter, and that is what makes the shortcut exact. Had the two words differed in two letters, the two spare digits could pair either way and a third word would be needed.
Key facts
- LIGHT is coded 25709 and SIGHT is coded 97065, and the two codes share 9, 7, 0 and 5.
- I, G, H and T are the shared letters, so the leftover pairing is L = 2 and S = 6.
- Matching by position is impossible here, because it would make I both 5 and 7.
Study next
Common traps
- Pairing letters to digits by position, which makes I both 5 and 7.
- Assuming the code for S sits in the place S occupies inside SIGHT.
- Solving for all five letters when the question asks for one.
Built the same way at 18 Sep 2024, 09:00, Reasoning Q.9 (DRIVE 62749, RIDES 76284, code for V): the words share four letters, and the digit left over in DRIVE's code, 9, is V.
At 10 Sep 2024, 12:30, Reasoning Q.16 (COWS 7935, OILY 2568, code for O) the logic runs the other way: the words share only O and the codes share only 5, so the shared letter takes the shared digit.
Related PYQs
No directly related past PYQ was found.