A pulverised coal sample passing a 200-mesh screen has a nominal particle size of 0.074 mm (74 microm). What is the approximate nominal particle size of material that just passes a 50-mesh screen?

Introduction / Context:
Screen mesh numbers denote openings per linear inch. Approximate relationships allow quick estimation of nominal aperture sizes to translate between mesh counts and particle sizes in classification tasks.

Given Data / Assumptions:

• 200 mesh ≈ 74 microm (0.074 mm).
• We seek the size for 50 mesh.
• Use standard sieve series approximations.

Concept / Approach:
As mesh decreases (fewer openings per linear inch), aperture increases. A common reference: 50 mesh ≈ 297 microm, i.e., about 0.297 mm. Rounding to two significant digits yields 0.30 mm.

Step-by-Step Solution:
Recognise 200 mesh → 0.074 mm (given).Recall 50 mesh aperture ≈ 0.297 mm.Select the closest option: 0.30 mm.

Verification / Alternative check:
ASTM E11/ISO sieve tables list 50 mesh near 300 microm.

Why Other Options Are Wrong:
0.007 and 0.014 mm: too fine for 50 mesh; those correspond to very high mesh counts.50 mm: off by orders of magnitude.

Common Pitfalls:
Confusing linear inch basis with square inch count; mesh is per linear inch.

Final Answer:
0.30 mm