If in a pond there were 20 lotus plants last year and through reproduction 8 new plants are added, taking the current population to 28, the birth rate is:
Answer & explanation
Correct answer: option 4
The correct answer is Option (4) → 0.4 offspring per lotus per year
Birth rate is the number of births in a given period counted per member of the starting population. It is a rate, so it is found by dividing the new individuals by the number present at the start.
Plants at the start (last year): 20
New plants added by reproduction: 28 − 20 = 8
Birth rate = 8 ÷ 20 = 0.4 offspring per lotus per year
Option (1) 1.4 offspring per lotus per year — Incorrect. This is 28 ÷ 20: the whole present population divided by the old one, not the births.
Option (2) 0.8 offspring per lotus per year — Incorrect. This doubles the right value; 8 ÷ 20 is 0.4, not 0.8.
Option (3) 2.5 offspring per lotus per year — Incorrect. This is 20 ÷ 8, the division turned upside down.
Option (4) 0.4 offspring per lotus per year — Correct. Birth rate is the number of births per individual in a given period. 8 new plants ÷ 20 plants = 0.4 offspring per lotus per year.
Note: A similar question was also asked in CUET (UG) 2026 on 19-05-2026 (Slot 2), and in CUET (UG) 2025 on 22-05-2025 (Slot 2) and on 15-05-2025 (Slot 1).
NCERT Extract:
Population Attributes – “… A population has certain attributes whereas, an individual organism does not. An individual may have births and deaths, but a population has birth rates and death rates. In a population these rates refer to per capita births and deaths. The rates, hence, expressed are change in numbers (increase or decrease) with respect to members of the population. Here is an example. If in a pond there were 20 lotus plants last year and through reproduction 8 new plants are added, taking the current population to 28, we calculate the birth rate as 8/20 = 0.4 offspring per lotus per year.”