Problem:
Compute the sum of the positive divisors (including 1) of that have units digit .
Solution
Solution:
The prime factorization of is . Every divisor of has prime factorization , where , , , and . If the divisor has units digit , it cannot be divisible by or , so .
Now take cases on the value of :
- If , then the divisor is for some . The possible divisors are , , , , and , of which and work.
- If , then the divisor is for some . The possible divisors are then , , , , and . Of these, only works.
The answer is .
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