Hesabu 19 : 12 Numbers chapter 19 verse 12

naye atajitakasa kwa yale maji siku ya tatu, na siku ya saba atakuwa safi; lakini kama hajitakasi siku ya tatu, ndipo na siku ya saba hatakuwa safi.
soma Mlango wa 19

the same shall purify himself therewith on the third day, and on the seventh day he shall be clean: but if he doesn't purify himself the third day, then the seventh day he shall not be clean.