The magnetic field of Earth can be modelled by that of a point dipole placed at the centre of the Earth. The dipole axis makes an angle of 11.3° with the axis of Earth. At Mumbai, declination is nearly zero. Then,

Explanation: The magnetic field lines of the Earth resemble that of a hypothetical magnetic dipole located at the centre of the Earth.
The axis of the dipole does not coincide with the axis of rotation of the Earth and it is tilted at some angle (angle of declination).
In this situation, the angle of declination is approximately 11.3° with respect to the later. So, there is two possibilities arises as shown :

So that the declination varies between. 11.3°W to 11.3° E.