Step 1: What Section 57 states.
Section 57 does not define life imprisonment; it only provides a conversion measure stating that life imprisonment shall be deemed equivalent to 20 years for calculating fractions of punishment.
Step 2: Judicial interpretation.
Courts have consistently held that life imprisonment means imprisonment till the last breath, but for remission calculations, 20 years is used.
Step 3: Conclusion.
Thus, when computing remission under Section 57, the law applies a fixed equivalent of 20 years.