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Explanation:

Let the male and female population be ‘m’ and ‘f’ in 1970. 

Using alligation

mf = 25-2040-2513

∴ f = 3m

Total population in 1970 = f + m = 4m

⇒ Number of females in 1980 = 1.2f = 3.6m and number of males in 1980 = 1.4m

Population in 1990
Number of females = 1.25 × 3.6m = 4.5m
Number of males = ½ × 4.5m = 2.25m
Total population = 4.5m + 2.25m = 6.75m

⇒ Total population increases from 4m in 1970 to 6.75m in 1990.

∴ Percentage increase = 2.75m/4m×100 = 68.75%.

Alternately,
Let the number of males and females in 1970 be 'm' and 'f' respectively.

∴ In 1980, number of males = 1.4m, females = 1.2f and total population = 1.25(m + f)
⇒ 1.4m + 1.2f = 1.25(m + f)
⇒ 0.15m = 0.05f
⇒ m : f = 1 : 3

∴ In 1990, number of females = 1.25 × 1.2f = 1.5f, number of males = 1/2 × 1.5f = 0.75f 
∴ Total population in 1990 = 1.5f + 0.75f = 2.25f

Also, total population in 1970 = m + f = f + f/3 = 4f/3

⇒ Required percentage = 2.25f-4f/34f/3 × 100% = 1116 × 100% = 68.75%

Hence, option (c).

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