Question

The average age of group of six girls is 12 years 10 months. One leaves the group and the average is decreased by 12 months.

How old is the girl who leaves the group?

Answer 1

Hi, the average is calculated in the first group of six girls with the next formula
TOTAL_AGE_6 / 6 = 12y 10 m = 12.833333
TOTAL_AGE_6 = 6(12y 10m)
TOTAL_AGE_6 = 72y 60m = 77y
The 6 girls acumulated total age is 77 years

One leaves the group, the new average with 5 girls is
TOTAL_AGE_5 / 5 = 12y
TOTAL_AGE_5 = 5*12y = 60y 
The 5 girls acumulated total age is 60 years

77 years - 60 years = 17 years
The girl who leaves the group is 17 years old