130. How many four-digit numbers can be formed from the digits $0,1,2,3,4,5$, if the numbers must be odd and digits may be repeated?
S o l u t i o n. For the first position, we can choose any of the five digits (excluding zero). For the second and third positions, we can choose any of the six digits, as digits can be repeated. For the last position, we can choose any of the three odd digits. Therefore, the total number of four-digit numbers is $5 \cdot 6 \cdot 6 \cdot 3=540$

A n s w e r: 540.