Its diagram is as follows:
6 × 2 = 12
Husband 3 → 6 shares
Mother 1 → 2 shares
Grandfather 1 → 2 shares
Two brothers 1 → 2 shares (1 each)

Fourth case: the grandfather’s muqāsama share and one-third of the remainder are identical, and both are more advantageous for him than a fixed one-sixth of the estate. An example is a mother, a grandfather, and two brothers, a configuration in which the allotted shares come to less than one-half of the estate and the brothers are twice the grandfather in number.

Why muqāsama and one-third of the remainder coincide is this: after the mother’s sixth has been deducted, five parts are left for the grandfather and the two brothers. One-third of that remainder equals one and two-thirds, precisely the amount he would take under muqāsama. Because a third of the remainder does not produce a whole number, we multiply three—the denominator required to extract a third—by the original base of the problem, six, yielding a new denominator of eighteen.

The final distribution is therefore:
Mother: her original one part becomes 3 shares, leaving 15.
Grandfather: 5 shares, whether by muqāsama or as one-third of the remainder.
Brothers: 10 shares in total, 5 shares apiece.