Question: What is the difference between 2\*a
and 2a?
What is the difference between 2(3+4)
and 2\*(3+4)?

We all know that omitting the multiplication sign still means multiplication so nothing changed!

This question is related to a well-known mathematical debate as follows:
6/2(1+2)
If we use the rule of order of operation which states that :
Parentheses first, exponents next, multiplication and division from left to right and finally addition and subtraction from left to right
The result will be 9
However, some would say that 6/2(1+2) is like
6/(2(1+2))

Having mentioned that they would say what is the result of 9a^2/3a?

I know this question might be "duplicate" or whatever...
But I really searched a lot and didn't find a satisfying answer.

Is 3a the same as 3\*a or (3\*a)?

So 9a^2/3a = (9\*a^2) / (3\*a)
Or 9\*a^2/3\*a

Answer: 
The difference between any two integer solutions is an integer multiple of the cross product $(3,-4,1).$ By inspection, I can see that $(0,20,0)$ is a solution. Next
$$ (3, 16,1), $$
$$ (6, 12,2) $$
$$ (9, 8,3) $$
$$ (12, 4,4) $$
$x+2y+5z=40$ and $x+y+z=20$ and $x,y,z > 0$

Subtracting you get $y + 4z = 20$, that is $y = 20-4z$ 

$y>0$ and $z>0$ implies $z \in \{1,2,3,4\}$.

Substituting $y=20-4z$ into $x+y+z=20$, we get $x=3z$.

So $x = 3z, \ y = 20-4z$, and $z \in \{1,2,3,4\}$.