Consider a 4-step process that begins with Operation 1, followed by Operation 2, Operation 3, and Operation 4 sequentially. The effective capacity of each operation is given in the following:

1) Operation 1: 20 units/hour;

2) Operation 2: 15 units/hour;

3) Operation 3: 18 units/hour;

4) Operation 4 : 12 units/hour.

What is the capacity of this process (expressed as units/hour)?