how to know when to multply or add subtract etc

Hi
So far I understans that if x is right up against a number it means to multiply. Then I watched a video where the guy said 'if you don't see a sign on a term then it means it's automaticly positive so 9y is = +9y this was the equation

9y-30=2y=5

but could he have used x instead of y? and if so how do you know if that 9y or 9x is plus or multiply?

Solution

Here you go. Go and take a look at the videos Ep 1 - 4 on Linear Equations. http://www.youtube.com/channel/UCF7v98NJfon7B8cpQ_0Ft4w/videos

Hope it helps.

I'm not sure what your question is. "9y" means "9 multiplied by y". As far as that is concerned, there is no question of "adding" anything. "9y- 30" says multiply 9 by y, then subtract 30 from that. Perhaps you are confusing a number being "positive" with addition. The fact that we use "+" in two different ways is a historical artifact. Whether you have "9y- 30" or "+9y- 30" they both means exactly the same thing: multiply 9 by y then subtract 30 from that.

I am also not sure what you mean by "could he have used x instead of y?". If it were just a matter of a single formula or equation, you can use whatever letter or even other symbol to indicate a variable. But if a problem has "x" in one place and "y" in another you have to keep track of which is which. That is, if an equation says "9y- 30= 2y- 5" or "9x- 30= 2x- 5" those are exactly the same. But if it says 9y- 30= 2x- 5, you cannot replace one with the other.