Part C is where CSIR NET Earth Science candidates lose the most marks — not because the concepts are unusually hard, but because most preparation focuses on Part B-style factual recall and leaves numerical problem-solving for the last few weeks. This guide works through representative solved numericals across the topics that come up most often, plus a practice framework for the rest.
Why Part C Numericals Decide Your Rank
Part C questions are negatively marked like the rest of the paper, but they carry more marks per question and reward speed under exam conditions. Candidates who can solve a numerical in ninety seconds instead of four minutes gain time across the whole section — and that time compounds into attempting more questions overall.
Worked Examples by Topic
1. Radiometric Dating (Isotope Geochemistry)
Problem: A rock sample contains a parent-to-daughter isotope ratio showing that 25% of the original parent isotope remains. If the half-life of the isotope is 1.3 billion years, what is the approximate age of the rock?
Solution: 25% remaining means the sample has gone through 2 half-lives (100% → 50% → 25%). Age = 2 × 1.3 billion years = 2.6 billion years.
2. Epicentral Distance (Seismology)
Problem: The time difference between P-wave and S-wave arrival at a seismic station is 4 minutes. Using an approximate S-P travel time constant of 8 km/second-difference, estimate the epicentral distance.
Solution: Epicentral distance ≈ S-P time (in seconds) × 8 km/s = 240 × 8 = 1,920 km (approximate, using the simplified Omori-style relation used in most CSIR NET-level problems).
3. Ocean Salinity and Density (Physical Oceanography)
Problem: Explain, without exact figures, why a water mass with higher salinity but the same temperature as another will always be denser.
Solution: Density in seawater increases with both decreasing temperature and increasing salinity. At constant temperature, dissolved salts add mass without proportionally increasing volume, so higher salinity directly increases density — this is the basis of thermohaline circulation questions that appear repeatedly in Part C.
4. Adiabatic Lapse Rate (Atmospheric Science)
Problem: A parcel of dry air rises from sea level. Using the dry adiabatic lapse rate of approximately 9.8℃ per km, what is its temperature after rising 2 km, if it started at 25℃?
Solution: Temperature drop = 9.8 × 2 = 19.6℃. Final temperature = 25 − 19.6 = 5.4℃.
5. Stratigraphic Thickness and Dip (Structural Geology)
Problem: A bed has a true thickness of 50 m and dips at 30°. What is its apparent thickness measured vertically?
Solution: True thickness = apparent (vertical) thickness × cos(dip). So vertical thickness = true thickness / cos(30°) = 50 / 0.866 ≈ 57.7 m.
How to Build Your Own Set of 50
Rather than memorizing worked answers, build a numerical practice log across these five categories, aiming for 8–10 solved problems each:
- Isotope geochemistry and radiometric dating
- Seismology (travel-time, magnitude, epicentral distance)
- Physical and chemical oceanography (density, salinity, circulation)
- Atmospheric thermodynamics (lapse rates, stability, humidity)
- Structural geology (dip/strike geometry, stress-strain problems)
This mirrors the actual spread of Part C numericals far better than cramming random previous-year answers without understanding the underlying formula.
Practice With Structured Material
The CSIR NET Earth Science Combo Course includes 250+ solved numerical problems organized by these same categories, alongside 25 full mock tests to practice under timed conditions. If you specifically want previous-year numerical patterns, see the CSIR NET Earth Science PYQ course.
Frequently Asked Questions
How many numericals typically appear in Part C?
The exact count varies by session, but numerical and diagram-based questions make up a large share of Part C — roughly 70% by most previous-year analyses.
Is a calculator allowed in CSIR NET?
No physical or app-based calculator is provided; all numericals are designed to be solvable with basic arithmetic and the formulas you’re expected to know.
Which topic has the most numerical-heavy questions?
Isotope geochemistry, seismology, and physical oceanography tend to have the highest concentration of numerical problems relative to their overall syllabus weight.