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\(45 = 5 × 3^2\)
We are given that the sum of all positive divisors of 45 is 78.
Now,
\(720 = 2^4 × 45\)
Every divisor of 720 can be formed by taking a divisor of 45 and multiplying it by one of the following:
\(1, 2, 2^2, 2^3\), or \(2^4\)
Therefore, the sum of all positive divisors of 720 is:
\(78 + 2(78) + 2^2(78) + 2^3(78) + 2^4(78)\)
\(= 78(1 + 2 + 4 + 8 + 16)\)
\(\\
= 78(31)\\
= 2418\\
\)

Answer: C
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