1. Let's define variables for the populations in 1970: - Let the male population in 1970 be $M$. - Let the female population in 1970 be $F$. 2. We know the total population in 1970 is $T = M + F$. 3. From 1970 to 1980: - Total population increased by 25%, so total in 1980 is $$ T_{1980} = T \times 1.25 = 1.25(M+F) $$ - Male population increased by 40%, so male in 1980 is $$ M_{1980} = M \times 1.40 = 1.4M $$ - Female population increased by 20%, so female in 1980 is $$ F_{1980} = F \times 1.20 = 1.2F $$ 4. Check total consistency in 1980: $$ M_{1980} + F_{1980} = 1.4M + 1.2F = 1.25(M+F) $$ Which simplifies to: $$ 1.4M + 1.2F = 1.25M + 1.25F $$ Rearranged: $$ 1.4M - 1.25M = 1.25F - 1.2F $$ $$ 0.15M = 0.05F $$ $$ 3M = F $$ So the female population in 1970 is three times the male population. 5. From this ratio, express $F=3M$. Total population in 1970: $$ T = M + F = M + 3M = 4M $$ 6. From 1980 to 1990: - Female population increased by 25%, so female in 1990 is: $$ F_{1990} = 1.25 \times F_{1980} = 1.25 \times 1.2F = 1.5F $$ - Male population in 1990 is unknown, denote as $M_{1990}$. 7. Given in 1990: $$ F_{1990} = 2 \times M_{1990} $$ Substitute $F_{1990} = 1.5 F$ and $F=3M$ from step 4: $$ 1.5F = 2 M_{1990} $$ $$ 1.5 \times 3M = 2 M_{1990} $$ $$ 4.5M = 2 M_{1990} $$ $$ M_{1990} = \frac{4.5M}{2} = 2.25M $$ 8. Calculate total population in 1990: $$ T_{1990} = M_{1990} + F_{1990} = 2.25M + 4.5M = 6.75M $$ 9. Recall total population in 1970 was $4M$. Calculate percentage increase from 1970 to 1990: $$ \text{Percentage increase} = \frac{T_{1990} - T}{T} \times 100 = \frac{6.75M - 4M}{4M} \times 100 = \frac{2.75M}{4M} \times 100 = 0.6875 \times 100 = 68.75\% $$ **Final answer:** 68.75% Therefore, the correct option is B) 68.75.