x = number of 1-cent stamps
y = number of 8-cent stamps
z = number of 12-cent stamps
We have 31 stamps all together, so x+y+z = 31.
"I have 4 more 1-cent stamps than 8-cent stamps" means we have the equation x = y+8. Whatever y is, add 8 to it to get x. Solve for y to get y = x-8.
You also have "twice as many one cent stamps as 12 cent stamps", so x = 2z. Solving for z gets you z = 0.5x
-------------
x+y+z = 31
x+x-8+z = 31 ... y replaced with x-8
x+x-8+0.5x = 31 ... plug in z = 0.5x
2.5x-8 = 31
2.5x = 31+8
2.5x = 39
x = 39/2.5
x = 15.6
Your teacher made a typo somewhere because we should get a positive whole number result for x (since x is a count of how many 1-cent stamps we have).
Answer:

Step-by-step explanation:
We can use either angle, but I'm going to use the one on the bottom. So, in order to find x, we need to use tangent. One side we know is the adjacent, and the side we don't know is the opposite, therefore we need tangent. Here's the equation:

Obviously, we can't have a root in our denominator, so we need to get rid of it somehow. Here's how:
We multiply the denominator of the fraction by
.
multiplied by itself is simply 2. Try it! We also want to multiply the numerator by
, but there isn't really a number we can use with that, so we'll just add it to the side. The equation you have now is:

Let's try to work this out now. Since the denominator is 2, we have to multiply both sides by it to find x.


We can plug 2 in for the x in the numerator now:

2 and 2 cancel out, so you get 1 in both the numerator and denominator. That's how we get our answer of 
Also, because this is a 45-45-90 triangle, you don't really have to do all that work. If it's a 45-45-90 triangle, both legs should be the same length. :)
Answer:
The graph of the equation 40.51x+12.45y=666.64 is attached with the answer where the horizontal axis represents the X axis and the vertical axis represents Y axis.
To plot the graph physically just find two points lying on the line. Mark the points on the graph sheet and then join them. This will give you the line represented by the equation.
To find points on the line assume the value of any one variable, substitute it in the equation, then solve the equation to find the value of other variable. For example : assume y = 1; substitute the value of y in the equation;
⇒ 40.51x + 12.45×1 = 666.64
⇒ 40.51x = 666.64 - 12.45
⇒ 40.51x = 654.19
⇒ x = 
⇒ x ≈ 16.149
Therefore point ( 16.149 , 1 ) lie on the graph of the equation.
***Only two points are required to plot this graph just because it represents a straight line, that we can conclude just by observing the equation. If in an equation the power of x is 1 or 0 and power of y is 1 or 0 then only it will represent a straight line in 2-D plane.***
Let be "x" the number of colored pencils in the box.
According to the information given in the exercise, there are 84 pencils in the box and 53 of them are regular pencils.
Knowing this information, you can set up the following equation:

Finally, you have to solve for the variable "x" in order to find its value. In order to do this, you can apply the Subtraction property of equality by subtracting 53 from both sides of the equation:

The answer is: There are 31 colored pencils in the box.