Ku-tiirsaneyaasha ururka {a.b.c} waxaad u ratibi kartaa lix siyaabood oo kala duwan: Rattibaad kasta waxay kaga duwantahay tan kale horsiimada ku-tiirsanayaasha. Raabaqaad waa rattibaad kasta oo ku-tiirsaneyaasha urur la siinayo horsiimo huban. Abnaqanka n oo loo qoro n! waa taranka tirsiimada laga bilaabo 1 ilaa n taasoo ah: n! = 1.2.3.4.... (n----3) (n----2) (n----1) n. Matalan 5! = 1.2.3.4.5. = 120 Ogoow 0! = 1, sidoo kale 1! = 1 Sumadda n P r {\displaystyle n{P}r} waxay u taagantahay raabaqaad n shey oo markiiba la qaato r shey sida, 8 P 3 {\displaystyle 8{P}3} waa tirada raabaqaad 8 shey oo markiiba la qaat qaatay 3 shey. Tirada raabaqaad n shey oo kala duwan oo markiiba r shey la israacsho waxay la mid tahay n P r {\displaystyle n{P}r} n (n -- 1) (n -- 2) . . . (n-- r + 1) = : n ( n − r ) !