Which decade registered the maximum growth rate (%) of population ?

Concept: The percentage growth rate of a quantity over a period is (increase during the period ÷ value at the start of the period) × 100. Because the starting value sits in the denominator, an identical absolute increase produces a larger percentage when it is measured on a smaller base.

Applying it to this table: Read the population column at the two endpoints of each decade offered, take the increase over that decade, and divide it by the population at the start of that decade.

1. 1961-71: the population goes from 21 million to 24 million, an increase of 3 million on a starting value of 21 million, so the growth rate is 3/21 = 1/7 ≈ 14.29%.

2. 1971-81: the population goes from 24 million to 27 million, an increase of 3 million on a starting value of 24 million, so the growth rate is 3/24 = 1/8 = 12.5%.

3. 1991-2001: the population goes from 30 million to 32 million, an increase of 2 million on a starting value of 30 million, so the growth rate is 2/30 = 1/15 ≈ 6.67%.

4. 2001-2011: the population goes from 32 million to 35 million, an increase of 3 million on a starting value of 32 million, so the growth rate is 3/32 = 9.375%.

Cross-check: Three of the four decades offered share the same absolute increase of 3 million, so their ranking is settled entirely by the base they are measured against: the smaller the starting population, the larger the percentage. Since 21 < 24 < 32, the decade starting at 21 million leads that group, and the remaining decade pairs the smallest increase (2 million) with a large base (30 million).

Every decade in the table, for comparison

Across the whole table the largest percentage growth of population is 14.29%, recorded in the decade 1961-71. Reading “growth rate” as the absolute rise instead would leave the question unanswerable, because four separate decades tie at an increase of 3 million; the percentage reading is what produces a unique answer.