Step 1: Identify the governing law.
Paid leave entitlement for mine workers in India is fixed by Section 52 of the Mines Act, 1952, and this section actually sets two different rates depending on where the person works.
Step 2: Note the two separate rates.
A person employed below ground earns leave faster, at the rate of one day for every 15 days worked, because underground work is considered more arduous, while any other person, meaning anyone working on the surface or in an opencast mine, earns leave at a slightly slower rate of one day for every 20 days worked.
Step 3: Apply this to the person in the question.
Since the question specifically asks about a person working in an opencast mine, which counts as surface work rather than below ground work, the applicable rate is the second one, not the below ground rate.
Step 4: Conclude.
So an opencast mine worker earns leave with wages at the rate of one day for every 20 days of work performed.
\[ \boxed{\text{One day for every 20 days of work performed by him}} \]