x.... why do we use x in Maths?

In Maths when we want to use a letter to stand for a number that we don't necessarily know yet or that we want to use different values for... we often use the letter x ... for example... the cooking time for a chicken is 40 minutes for every kilo, so the time it takes depends on how heavy the chicken is... 
We could turn this into a formula where  x  is the weight (in kg) of the chicken  Time = 40x
This is what we often mean by algebra

So...
when x = 1 the cooking time is 40 x 1 = 40 minutes
when x = 2 the cooking time is 40 x 2 = 80 minutes
when x = 3 the cooking time is 40 x 3 = 120 minutes
when x = 4 the cooking time is 40 x 4 = 160 minutes
when x = 5 the cooking time is 40 x 5 = 200 minutes
....

The formula Time = 40x   saves us from having to keep writing this out lots of times!

But why x?

There are a few theories however a popular one is that back in the 17th Century in France a famous philosopher and mathematician René Descartes was sending a lot of maths to get get printed. In order to print a book the printers had to use individual blocks of wood with each letter. In René's maths he used lots of different letters to stand for the 'missing numbers' and the printers suggested that in the French language they didn't use many x's so they had plenty of them they could use.... so as a result, all of René's 'missing numbers' are shown using x.

René Descartes also introduced the idea of using (x,y) coordinates and graphs

Click here to read more about the history of algebra and here