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It follows exponential distribution with parameter 1/5. So, I guess 0.1638 is the correct answer.
How to know it follows exponential distribution? I guess in GATE they will mention it.

$\lambda = \text{ number of calls in 1 minute} = \frac{1}{5} = 0.20$

$P(X \geq x) = e^{ - \lambda x}$

$P(X < x) = 1 - e^{ - \lambda x}$

So,

$P(X<1) = 1 - e ^{-0.2 \times 1} = 0.18126$

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