Praise be to Allaah.
She has to work out the number of days when she did not
fast according to what she thinks is most likely, and fast them with the
intention of making them up, until she is certain that she has made up all
the days that she owes. As well as that, she should feed one poor person
for each day, so as to be on the safe side and to avoid an area of
scholarly difference of opinion, if she is able to feed others.
For more information on feeding the poor, please see the
answer to question no. 26865.
If she is fasting on Mondays and Thursdays, then she should
intend that to make up what she owes, so that she will complete the days
that she owes.
Shaykh Saalih al-Fawzaan was asked: If a girl reaches the age
of puberty before the month of Ramadan begins, but she does not fast that
year in which she reached puberty because she is not aware that fasting is
obligatory for her, what does she have to do? Is the ruling in this case the
same as the ruling as one who breaks the fast deliberately with no excuse?
He replied: She has to repent to Allah, may He be glorified
and exalted, and make up the same number of fasts as the days that she
missed, and she also has to feed one poor person for each day, if the next
Ramadan came and she had not fasted those days. End quote.
Al-Muntaqa fi Fataawa al-Fawzaan,
81/11
And Allah knows best.