CSAT Solved Papers/ 2021/Q28

2021 CSAT — Q28

Quant Logical & quantitative reasoning 2.5 marks Medium

You are given two identical sequences in two rows:

Sequence-I	$8$	$4$	$6$	$15$	$52\cdot 5$	$236\cdot 25$
Sequence-II	$5$	$A$	$B$	$C$	$D$	$E$

What is the entry in the place of $C$ for the Sequence-II?

A 2.5
B 5
C 9.375 Answer
D 32.8125

Worked rationale

Find the rule from Sequence-I by taking ratios of consecutive terms:

8\to4\ (\times 0.5),\quad 4\to6\ (\times 1.5),\quad 6\to15\ (\times 2.5),\quad 15\to52.5\ (\times 3.5),\quad 52.5\to236.25\ (\times 4.5).

The multipliers rise by $1$ each step: $0.5,1.5,2.5,3.5,4.5$ . Apply the same multiplier chain to Sequence-II starting at $5$ :

A = 5\times 0.5 = 2.5,\quad B = 2.5\times 1.5 = 3.75,\quad C = 3.75\times 2.5 = 9.375.

$C$ is the $4$ th term.

Answer: (c) 9.375.

Why the other options miss

A

answered the sub-step, not the question: stops at $A$ (the $2$ nd term) instead of advancing to $C$ (the $4$ th).
B

reports the seed: gives the starting value of Sequence-II rather than its $4$ th term.
D

one term too far: overshoots to $D$ ( $C\times 3.5$ ), one term past the one asked for.

Specialist insight

“Identical sequences” means identical multiplier rule, not identical values — so extract the ratio chain ( $\times 0.5,1.5,2.5,3.5,4.5$ ) once and replay it on the new seed. The two live traps are positional: $C$ is the $4$ th letter ( $5,A,B,C$ ), so apply exactly three multipliers ( $0.5,1.5,2.5$ ), no more, no fewer. Track the term index on your fingers to avoid stopping at $A$ or running on to $D$ .

The trap, in one line

Multipliers $0.5,1.5,2.5$ on $5$ give $C=5\cdot 0.5\cdot1.5\cdot2.5=9.375 \Rightarrow$ (c).

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