Population growth sum: births, deaths, immigration, emigration. My answer doesn't match the key
Kavya NaikDropper Sample
13 Aug 2026 · 1 min read
Amma says I should solve at least one numerical every morning with chai, so here's the one that broke me today.
A population of 50 individuals in a pond. During a month: 10 births, 5 deaths, 3 immigrants, 2 emigrants. Find the population at the end of the month.
NCERT gives the idea that population density at time is
I wrote 50 + (10 + 3) - (5 + 2) = 56. The key says... 56. Fine, that part was OK! But the very next sub-question asks for the birth rate and the death rate and my answer for those didn't match: I wrote 10 and 5. Key says 0.2 and 0.1 per individual per month.
I understand per capita (per head) now, but then I realised I don't know when to use "birth rate" vs "natality" and what is the difference between "natality" and "birth" too.
And in the same set, there's a question on age pyramids. I mixed up declining vs expanding pyramids. Which one has the wide base? Also is the human population pyramid in India closer to expanding or stable? I'm not sure NCERT says.
Sorry it's a long one, I will mark all the answers as helpful 🙏
8 answers & replies
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[+][–]Ishita GoyalDropperSample· 2mo ago
Natality = number of births during a given period in the population, expressed per unit of population (per capita per time), so as a rate. "Births" = the raw count (10). Rate = births / initial population = 10/50 = 0.2 per individual per month. Death rate (mortality) = 5/50 = 0.1. Same logic for immigration and emigration if they ask. Always divide by the starting population, not the final one.
[+][–]Kavya NaikDropperSample· 2mo agoAuthor
Divide by the starting population, finally. Thank you Ishita!
[+][–]Nisha Verma12thSample· 2mo ago
Age pyramids, my table:
- Expanding: wide base, narrow top (triangle), many pre-reproductive individuals, population growing
- Stable: bell-shaped, roughly equal pre-reproductive and reproductive
- Declining: narrow base, urn-shaped, fewer pre-reproductive individuals
Trick: the base is the future. If the base is wide, lots of youngsters are coming into the reproductive group, so growth.
[+][–]Rahul DubeyDropperSample· 2mo ago
Declining pyramid has a wide base because lots of births, right? Then they all die young?
[+][–]Siddharth RaoDropperSample· 2mo ago
Rahul, no. Declining = narrow base. Wide base is expanding. For a declining population births are low, so there are fewer young individuals than middle-aged ones, which gives the urn/bulb shape. If a wide base also had lots of deaths, it's still an expanding pyramid with high mortality, not declining.
[+][–]Rahul DubeyDropperSample· 2mo ago
Ah yes, got my two ends mixed up in a long day. Thanks Siddharth.
[+][–]Pooja Iyer12thSample· 2mo ago
For India, I don't think NCERT claims anything specific so I would not write it in a theory answer. Just know the shapes. Also fun: for population density NCERT says to remember it's not always a head-count. For a big banyan tree or for a bunch of algae in a pond, the biomass or percent cover is a better measure than the number. 🌳
[+][–]Mehak AroraDropperSample· 2mo ago
Another detail that gets MCQs: the sex ratio. It's the ratio of males to females in a population, and a population can have different ratios at different ages. Not part of your sum but NCERT includes it under attributes, along with age distribution and population size or density.
