Can someone please tell me why the area was divided by two for the chair?

Can someone please tell me why the area was divided by two for the chair? (see full problem below) (4π (0.015 / 2 m)2)

A 70 kg student stands on a 10 kg chair. The chair has four cylindrical legs each with a diameter of 0.015 m. What pressure does the student and chair exert on the ground?

Answer:

P = F / A

The force is the total weight, and the area is the total cross-sectional area of the four chair legs.

P = mg / (4 πr2)

P = (70 + 10 kg)(9.8 N/kg) / (4π (0.015 / 2 m)2)

P = 1.1x106 Pa