Question

What is `x` when `y=18`, if `y=5` when `x=4`?

Answer 1

The question is quite incomplete, There may be several answers for it.

Explanation:

For example.,

Lets say `y = x + 1` is equation 1.
So here, when `x = 4, y = 5.`

Also, `y = 1.25 x`, is equation 2

Here also, when `x= 4 , y` = 5 ,

But these equations give different results when `y` = 18

For equation 1,

`18 = x + 1`
So, `x = 17`

For equation 2,

`18 = 1.25x`

`18/1.25 = x`

So, `x = 14.4`

Answer 2

Some rule or formula needs to be given to connect x and y.

Explanation:

In addition to what has been given by Anuj, here are other options for x and y if they are in proportion to each other:

Directly proportional:

`x/18 = 4/5`

`5x = 4xx18`

`x = (4xx18)/5`

`x = 14.4`

Inversely proportional:

`x xx y = 5xx4 = 20`

`x xx 18 =20`

`x = 20/18 = 10/9`

If they are not in proportion, some rule or formula needs to be given to connect x and y.