x = number of 15 cent stamps

0.15x = cost of 15 cent stamps bought

y = number of 25 cent stamps

0.25x = cost of 25 cent stamps bought

z = number of 45 cent stamps

0.45x = cost of 45 cent stamps bought

38 = total number of stamps bought = x + y + z

$9.50 = total cost of stamps = 0.15x + 0.25y + 0.45z

number of 25 cent stamps was 8 more than twice the number of 45 cent stamps, so 

y = 8 + 2z

8 = y - 2z

We have three simultaneous equations

9.5 = 0.15x + 0.25y + 0.45z (the first equation)

38 = x + y + z (the second equation)

8 = y - 2z (the third equation)

Let us first try to eliminate the x term by subtracting the first two equations. But first we have to do something to make the coefficients of the x terms equal. We have 0.15x and 1x. We can accomplish this by multiplying the second equation by 0.15

38 = x + y + z

(38)(0.15) = (x + y + z)(0.15)

5.7 = 0.15x + 0.15y + 0.15x

Now subtract this equation from the first equation above

9.5 = 0.15x + 0.25y + 0.45z

5.7 = 0.15x + 0.15y + 0.15z

--------------------------------------…

3.8 =.....0x.+...0.1y.+..0.3z

So the resultant equation is

3.8 = 0.1y + 0.3z

We now have two equations left

3.8 = 0.1y + 0.3z

8.0 = y - 2z

If we multiply the bottom equation by 0.1 then subtract the two equations, the y terms will cancel out so we could solve for z.

(8.0)(0.1) = (y - 2z)(0.1)

0.8 = 0.1y - 0.2z

Now we subtract the two equations

3.8 = 0.1y + 0.3z

0.8 = 0.1y - 0.2z

--------------------------

3.0 = 0.0y + 0.5z

3 = 0.5z

z = 6

Substituting 6 for z in 8.0 = y - 2z, we can solve for y.

8.0 = y - 2z

8 = y - 2(6)

8 = y - 12

y = 20

If we use the second original equation 38 = x + y + z, by substituting 6 for z and 20 for y, we can solve for x

38 = x + y + z

38 = x + 20 + 6

x = 12

So Andy bought 

twelve 15 cent stamps = 12 * 0.15 = $1.80

twenty 25 cent stamps = 20 * 0.25 = $5.00

six 45 cent stamps = 6 * 0.45 = $2.70

--------------------------------------…

Work the second problem using the same technique as this problem. I will help get you started.

The sum of the length, width, and height of a rectangular box is 75 cm. 

75 = length + width + height.

The length is twice the sum of the width and height. 

length = 2 * (width + height)

The width exceeds the height by 5 cm. 

width = height + 5