Solution

Let the sum of age of 19 girls be x years and the Age of the new girls be y years. Old Average = (Sum of the age of 19 girls + Age of the girl to be replaced)/Total number of girls => (x + 18) / 20 New Average = (Sum of the age of 19 girls + Age of the new girl) / 20 According to the question, As we know The average age of the girls is decreased by 2 months when one girl aged 18 years is replaced i.e. Old Averag - New Average = 2 month ⇒ {(x + 18)/20} – {(x + y)/20} = (2/12) ⇒ 18 – y = 10/3 ⇒ y = 44/3 years ⇒ y = 14 years 8 months ∴ The age of new girl is 14 years 8 months.