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imc

number theory intermediate

Problem

How many three-digit numbers are not divisible by , have digits that sum to less than , and have the first digit equal to the third digit?
(A)
(B)
(C)
(D)
Solution
We use a casework approach to solve the problem. These three digit numbers are of the form .( denotes the number ). We see that and , as does not yield a three-digit integer and yields a number divisible by 5. The second condition is that the sum . When is , , , or , can be any digit from to , as . This yields numbers. When , we see that so . This yields more numbers. When , so . This yields more numbers. When , so . This yields more numbers. When , so . This yields more numbers. Summing, we get
Final answer
B