A tapered circular rod of diameter varying from $20$ mm to $10$ mm is connected to another uniform circular rod of diameter $10$ mm as shown in the following figure. Both bars are made of same material with the modulus of elasticity, $E=2 \times 10^5$ MPa. When subjected to a load $P=30 \pi$ kN, the deflection at point $A$ is ______ mm.
