The period of S. H.M. of a particle is 16 second. The phase difference between the positions at $\text{t} = 2\text{ s}$ and $\text{t} = 4\text{ s}$ will be}
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Phase difference is just the fraction of the total cycle. $2 \text{ s}$ is $1/8$ of the $16 \text{ s}$ period. Since a full cycle is $2\pi$, $1/8$ of $2\pi$ is $\pi/4$.