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GATE 2023 CS – Question 3

General Aptitude · Analytical Aptitude: Logical Reasoning and Deduction · 1 mark · Multiple choice

A series of natural numbers $F_1, F_2, F_3, F_4, F_5, F_6, F_7, \ldots$ obeys $F_{n+1} = F_n + F_{n-1}$ for all integers $n \ge 2$. If $F_6 = 37$, and $F_7 = 60$, then what is $F_1$?

  1. 4
  2. 5
  3. 8
  4. 9

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Correct answer: (A) 4

Explanation

Work backwards: F5=60-37=23, F4=37-23=14, F3=23-14=9, F2=14-9=5, F1=F3-F2=9-5=4.