What is the input of the function?

What is the input of the function when the output is 20?

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It depends what the function is.

If the function is f(x) = 2x + 4

Then, to find out what the input is when the output is 20, substitute 20 in for f(x) and solve for x.

20 = 2x + 4 {substituted 20 in for f(x)}
16 = 2x {subtracted 4 from each side}
x = 8 {divided each side by 2}

The input would be 8, when the output is 20.....in this example.

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