Given set $A$ of all human beings in a town at a particular time and $R=\{(x,y):\; \text{x is exactly 7cm taller than y}\}$

If $x$ is exactly 7cm taller than $y$, $\rightarrow$ obviously $(x,x) \not \in R$, as $x$ cannot be 7cm taller han $x$.Hence $R$ is not reflexive.

If $x$ is exactly 7cm taller than $y$, $y$ obviously cannot be exactly 7cm taller than $x\; \rightarrow (x,y) \in R$, but $(y,x) \not \in R$. Therefore $R$ is not symmetric.

If $x$ is exactly 7cm taller than $y$, and $y$ is exactly 7cm taller than $z \; \rightarrow$ $x$ is obviously exactly 7+7 = 14cm taller than $z$ and not exactly 7cm taller than $z$.

Therefore, $(x,y) \in R, (y,z) \in R$ but $(x,z) \not \in R$. Hence $R$ is not transitive.