Question

The function f has the property that, for all x, 5f(x) = f(5x). If f(20) = 40, what is the value of f(4)?

Answer 1

Answer: 17

Explanation: To get a feel for this information, let's first pick a random value for x.
Let's say x = 2
The given info tells us that f(x) = f(x + 1) for all values of x
So, f(2) = f(2 + 1)
In other words, f(2) = f(3)

Let's try a different value of x.
How about x = 3
The given info tells us that f(x) = f(x + 1) for all values of x
So, f(3) = f(3 + 1)
In other words, f(3) = f(4)

And so on.

We can see that f(2) = f(3) = f(4) = f(5) and so on
So, if f(4) = 17, we can be certain that f(8) = 17 as well.