Discussion
What is the difference between the greatest 4-digit number and the least 4-digit number formed by using the digits 1, 5, 0 and 7 once?
Answer: C
The difference is 6453.
For the greatest number, arrange the digits in descending order: 7510. For the least 4-digit number, put the smallest non-zero digit first and then the rest in ascending order: 1057, because a number cannot begin with 0.
\[ \begin{aligned} 7510 - 1057 &= 6453 \end{aligned} \]
Using 0157 as the least number is not allowed, since it is only a 3-digit number. 6093, 6003 and 6531 come from other wrong choices of the two numbers or from slips in subtraction.
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