In a town 80% of the population are adults of which the men and women are in the ratio of 9 : 7, respectively. If the number of adult women is 4.2 lakh, what is the total population of the town?

let the number of adult men be N.
Then, ratio of the numbers of adult men and adult women
9 : 7 = N : 4.2
⇒ N = (9/7) x 4.2 = 5.4 Lakh
Total adult population = 4.2 + 5.4 = 9.6 lakh.
If the population of the town be M Lakh,
then, 80M/100 = 9.6
⇒ M = (9.6 x 100)/80 = 12 lakh