The relation R in R defined as R = {(a, b) : a ≤ b } is-

Reflexive and transitive 

R = {(a, b) : a ≤ b } 

Clearly , (a, a) ∈ R as a=a

so, R is reflexive.

Now, (2,4)∈ R (as 2<4)

But (4,2) ∉ R as 4 is greater than 2.

So, R is not symmetric.

Now, let (a, b). (b, c) ∈ R

then  a ≤ b and b ≤ c

⇒ a ≤ c

⇒ (a, c) ∈ R

So, R is transitive.

Hence R is reflexive and transitive but not symmetric.