If f(x) varies directly with x and f(x) = 3 when x = 12, then what is the value of f(x) when x = 40?

Could someone explain this to me? I got 4 as my answer.

Question

If f(x) varies directly with x and f(x) = 3 when x = 12, then what is the value of f(x) when x = 40? 

Could someone explain this to me? I got 4 as my answer.

anonymous · Answer

Didn't I just do this for you haha? Why do you say 4?

anonymous · Answer

I didn't see that you replied to it! Oops.

anonymous · Answer

The equation we use for such questions is f(x) = kx where k is a constant, you can use any letter it's a constant. So you just need to find this constant.

So what I did earlier,

$$f(x) = 3,~~~3=12k \implies k = 3/12 \implies 1/4 \implies f(x) = \frac{ 1 }{ 4 }x$$

anonymous · Answer

The only possible answers that are listed are  
480
160
10
4

anonymous · Answer

Oh sorry I missed the part that says when x = 40

anonymous · Answer

So what you have to do here is, we have 1/4, so we have f(x) = 1/4x so now if we have x = 40 we just plug in 40 where the x is, so we get f(x) 40/4 which is?

anonymous · Answer

10?

anonymous · Answer

Yes!

anonymous · Answer

Do you see what I did?

anonymous · Answer

Yes! It took me a minute, but it helped that you broke it down for me. Thank you so much!

anonymous · Answer

Np ^.^