How should a very old woman purify herself and pray, when she finds it extremely difficult to perform ablution, makes tayammum on a stone, and does not remember the number of rak'ahs?

Prayer is obligatory upon the mother as long as her intellect is sound. Whatever she is unable to fulfill of its conditions and pillars is waived for her. She can keep track of the number of rak'ahs by counting with her fingers or pebbles. She must perform wudu' if she is able, even if by paying a fee. Otherwise, she should perform tayammum (definition) or wash what she is able to wash and perform tayammum for the rest. It is permissible for her to combine the two prayers that share a time, either by advancing or delaying one, due to hardship.