My grandfather died without leaving a will. As a result, we do not know how much he owes in prayers. What should we do?
If it has not stipulated in his will then it is not obligatory upon you to make-up his missed prayers. However, if you choose to do so, you may choose the minimum number that you are sure that he missed and it is better for you to estimate the most that he may have missed. You may hire someone to perform the prayers on his behalf.