Tyre width - this is the distance in millimetres between the sidewalls, in this example 155 mm.
Tyre profile - this is the ratio (expressed as a percentage) of the height of the tyre to its width, 155/50 is 50% of 155, which means a tyre height of 77.5mm
The letter R or D indicates the type of tyre: R for Radial or D for Diagonal.
The rim size - indicates the diameter of the rim in inches, in the example 13 inches (1 inch is 2,54 cm).
From these data we will calculate the radius:
The radius of the rim is half the diameter i.e. 6.5 inches i.e. 6.5*2.54cm equal to 16.51cm
Tyre height is 50% of 15.5cm i.e. 7.75cm
This gives a wheel radius as the sum of the above values equal to 24.26cm when rounded to 243mm.