Have no clue where to start

Here is an assignment that I have been stuck on for about an hour. This is coding in C. Problem I'm having is reading an unknown number of values.

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Statisticians use many different algorithms in addition to the arithmetic average. Two other averages are

the geometric and the harmonic mean.

Write a program that reads a series of numbers and calculates the average, geometric mean, and harmonic

mean. Do NOT use arrays for the series of numbers. Utilize functions for modularity.

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I think I can totally manage this with using arrays, but without them I feel like I have no idea how to even start it. The functions and everything else will be easy to do, if I can find a way to read an unknown number of inputs. I couldn't think of any efficient ways to do so, maybe someone here could point me in the right direction. Thanks a lot!