Rub‘ ‘ushr of a musinnah = 1/40 of its value/share.
Nisf ‘ushr = 1/20, or 2/40.
Three quarters of one tenth = 3/40.
These are proportional additions beyond the main musinnah due at 40; they do not mean another entire musinnah becomes due at 41. The ordinary rules for valuing and discharging fractions may be relevant in practice.