Problem:
How many integers with four different digits are there between and such that the absolute value of the difference between the first digit and the last digit is
Answer Choices:
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Solution:
From the set there are sixteen pairs of numbers whose difference is . All but can be used as the first and last digit, respectively, of the required number. For each of the ordered pairs there are ways to fill the remaining middle two digits. Thus there are numbers of the required form.