Question 1099387
The youngest sister is {{{highlight(3)}}} years old.
Four years ago there were only 3 sisters, whose ages added up to 15.
Each of those sisters is 4 years older now,
so naturally, the sum of the ages of those 3 sisters has increased by {{{3*4=12}}}
to {{{15+12=27}}} .
A new sister was born, who is now {{{3}}} years old,
so now the sum of the sisters' ages is {{{27+3=30}}} .