51. How many four-digit numbers contain only one even digit?
51. Four-digit numbers, in which only the first digit is even, amount to $4 \cdot 5 \cdot 5 \cdot 5=500$. Four-digit numbers, in which only the second (third, fourth) digit is even, amount to $5 \cdot 5 \cdot 5 \cdot 5$. Then, the total number of four-digit numbers, in which only one digit is even, will be $4 \cdot 5 \cdot 5 \cdot 5+3 \cdot 5 \cdot 5 \cdot 5 \cdot 5=2375$.