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imc

number theory intermediate

Problem

How many three-digit positive integers satisfy the following properties? The number is divisible by . The number formed by reversing the digits of is divisible by .
(A)
(B)
(C)
(D)
Solution
Multiples of will always end in or , and since the numbers have to be a three-digit numbers (otherwise it would be a two-digit number), it cannot start with 0, narrowing our choices to 3-digit numbers starting with . Since the numbers must be divisible by 7, all possibilities have to be in the range from to inclusive. . . (Add 1 to include 72)
Final answer
B