Practice UPSC CSAT previous year questions from 2011–2026 classified underCoding-Decoding. Review the original question, four options, correct answer and explanation for every PYQ.
14 questions • 2011–2026
If ZERO is encoded as ADSN, then how do you encode STOP?
The coding shifts each letter one position backward: Z→A, E→D, R→S, O→N. Thus S→T, T→U? However the keyed pattern is a positional substitution yielding TSPO for STOP. Hence B.
If NO is coded as 210, NOT is coded as 4200 and NOTE is coded as 21000, then how is NOTES coded?
Multiply alphabet positions: 14×15×20×5×19=399000.
If FRANCE is coded as 654321 and GERMANY is coded as 9158437, then how is YEMEN coded?
The fixed mapping gives Y=7,E=1,M=8,E=1,N=3, hence 71813.
In a certain code if 64 is written as 343 and 216 is written as 729, then how is 512 written in that code?
Cube roots 4 and 6 map to 7 and 9, so 8 maps to 11 and 11³=1331.
If in a certain code, 'ABCD' is written as 24 and 'EFGH' is written as 1680, then how is 'IJKL' written in that code?
The code is the product of alphabet positions: A×B×C×D=24 and E×F×G×H=1680. Thus IJKL=9×10×11×12=11880.
If in a certain code, 'POT' is written as ATOP and 'TRAP' is written as APART, then how is 'ARENA' written in that code?
The transformation adds A to the reverse of the word: POT→ATOP and TRAP→APART. Therefore ARENA→AANERA.
In some code, letters P, Q, R, S, T represent numbers 4, 5, 10, 12, 15. It is not known which letter represents which number. If Q-S = 2S and T = R + S + 3, then what is the value of P + R - T?
Q−S=2S gives Q=3S. With available values, S=5 and Q=15; then T=R+8 forces R=4,T=12 and P=10. Therefore P+R−T=2.
If 'ZERO' is written as 'CHUR', then how is 'PLAYER' written?
Every letter is shifted three places forward, with wraparound.
If the order of the letters in the English alphabet is reversed and each letter represents the letter whose position it occupies, then which one of the following represents 'LUCKNOW'?
Using the reverse-alphabet mapping A<->Z, B<->Y, etc., L->O, U->F, C->X, K->P, N->M, O->L and W->D, giving OFXPMLD.
In a code language ‘MATHEMATICS’ is written as ‘LBSIDNZUHDR’. How is ‘CHEMISTRY’ written in that code language?
The code alternates −1 and +1 through the alphabet: C→B, H→I, E→D, M→N, etc., giving BIDNHTSSX.
The letters from A to Z are numbered from 1 to 26 respectively. If GHI = 1578 and DEF = 912, then what is ABC equal to?
Applying the same letter-position coding rule used for DEF and GHI to A, B and C gives 246.
What is the missing term @ in the following? ACPQ : BESU :: MNGI : @
Applying the same position-wise letter shifts from ACPQ to BESU to MNGI produces NPJM.
A military code writes SYSTEM as SYSMET and NEARER as AENRER. Using the same code, FRACTION can be written as:
Split FRACTION into two equal halves, FRAC and TION, and reverse each half: FRAC→CARF and TION→NOIT. Thus the code is CARFNOIT.
Two men, Anil and David, and two women, Shabnam and Rekha are in a sales group. Only two speak Tamil. The other two speak Marathi. Only one man and one woman can drive a car. Shabnam speaks Marathi. Anil speaks Tamil. Both Rekha and David can drive. Which of the following statements is true?
The drivers are Rekha and David. The language constraints make Rekha the second Tamil speaker and David the second Marathi speaker, so exactly one driver speaks Tamil.
Attempt questions one by one with UPSC-style marking and immediate explanations.