Assertion (A): The RMM of a gas is 64, then its vapour density is 32.

Reason (R): RMM is twice the vapour density.

1. Both A and R are true and R is the correct explanation of A.
2. Both A and R are true but R is not the correct explanation of A.
3. A is true but R is false.
4. A is false but R is true.

Both A and R are true and R is the correct explanation of A.

Reason — Vapour density (V.D.) is related to relative molecular mass by:

$\text{V.D.} = \dfrac{\text{ Relative molecular mass}}{2}$

Substituting the given value:

$\text{V.D.} = \dfrac{62}{2} = 32$

Thus the assertion is correct. Because the reason states the same relationship—“The RMM is twice the vapour density”—it correctly explains why the vapour density is 32.