Q.The population of a town was 8,00,000 in 2001 and 9,60,000 in 2011. Calculate the decadal population growth rate.

The decadal growth rate is found by comparing the increase in population to the earlier year's population, expressed as a percentage.

Given: Population in 2001 ($P_{t-1}$) = 8,00,000; Population in 2011 ($P_t$) = 9,60,000.

Formula:

Step 1 — Find the absolute increase:

$P_t - P_{t-1} = 9,60,000 - 8,00,000 = 1,60,000$

Step 2 — Apply the formula:

Independent verification (cross-check): $8,00,000 \times 1.20 = 9,60,000$, which exactly matches the given 2011 figure, confirming the computation.

An alternative check: since the population grew from 8,00,000 to 9,60,000, it grew by a factor of 9,60,000 / 8,00,000 = 1.20, i.e. a 20% increase — the same answer reached from the other direction, confirming the result.

A frequent error is dividing the increase by the LATER year's population instead of the earlier year's — the denominator must always be $P_{t-1}$ (the base/earlier year), not $P_t$.