25. How many even four-digit numbers can be formed from the digits $0,1,2,3,4,5$?
25. Since zero cannot be the first digit, the first digit can be chosen in five ways. The second and third digits can each be chosen in six ways. Finally, the fourth digit, which must be even, can be chosen in three ways. Then, by the rule of product, we will have $5 \cdot 6 \cdot 6 \cdot 3=540$ four-digit even numbers.