A mining operation extracts gold from a specific site. The mass of gold extracted in the first month is $a$ kg, and the mass extracted in each subsequent month is a constant fraction $r$ of the mass extracted in the previous month.

Given that:

• the total mass of gold extracted in the first three months is $74$ kg
• the total mass of gold that could theoretically be extracted if mining continued indefinitely is $128$ kg

find, for this operation,