Praise be to Allaah.
The precise rewards for the deeds which people do for the sake of Allaah is something which is known only to Allaah. If a person seeks the reward from Allaah and strives to obey Him, his reward will not be lost, as Allaah says (interpretation of the meaning):
We shall not make the reward of anyone who does his (righteous) deeds in the most perfect manner to be lost. [al-Kahf 18:30]. If someone has missed some of the days of Ramadaan, he should fast them first, then fast six days of Shawwaal, because he cannot follow the fast of Ramadaan with six days of Shawwaal unless he has completed his Ramadaan fast.
And Allaah is the source of strength. May Allaah bless our Prophet Muhammad and his family and companions, and grant them peace.
Reproduced from Islam QA