How does the strength of the electric field vary with the distance for the positive point charge?

It is known as Coulomb's law and is the equivalent in electrostatics of the Newton's law for gravity. This is a law falling in the "inverse square" category, meaning there is a relationship of the form 1 / (square n). When the distance is multiplied by n, the field is divided by square n, e.g. if the distance double, the field is divided by 4.

The exact formulation of Coulomb's law is:

E = 1 / (4 pi . epsilon0) . q / r2

E being the magnitude of the field, which is what you want to know. Unit is V/m.

epsillon0 being the electric constant (vacuum permittivity). Unit is C/V/m

q being the charge of the particle creating the field. Unit is Coulomb.

r being the distance from the charge. Unit is m.

The "inverse square" factor is q / r2

Coulomb's law is a special case of Gauss's law which is turn is included in the Maxwell's set of equations. It turns out that, in magnetism there are only two guys in charge... Maxwell and Lorentz. Kind of monopoly.