The average age of a group of 25 girls is 10 years. If one girl aged 12 years and 4 months joins the group, find, correct to one decimal place, the new average age of the group.

25 x 10 ==250 sum of all girls'ages.

New girl's age ==12 4/12==12 1/3 years.

250 + 12 1/3 ==262 1/3 - sum of the 26 girls' ages.

[262 1/3] / 26 ==10.1 - years - new average of the 26 girls.