A total of 15 chickens were sold by the farmer.

We know that the fourth customer bought just one from the total chickens. Thus, we can form the equation:

Total - (Total / 2 + 1/2) = 1

If you solve it, you will find that the total is 3.

Now, we can keep working this method till we reach the first customer.

The 1st customer bought 15 / 2 + 1/2 = 8 (leaving 7)

The 2nd customer bought 7 / 2 + 1/2 = 4 (leaving 3)

The 3rd customer bought 3 / 2 + 1/2 = 2 (leaving 1)

The 4th customer bought 1 / 2 + 1/2 = 1

When we add the numbers, we get

8 + 4 + 2 + 1 = 15.