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
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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).
The number of one-cent stamps is 14.
Total stamps = 31
We have three types of stamps: 1-cent stamps, 8-cent stamps, and 12-cent stamps.
The person has 4 more 1-cent stamps than 8-cent stamps.
The person has twice as many 1-cent stamps as 12-cent stamps.
We have to make equations with the given above statement.
Consider,
1-cent stamp = A
8-cent stamp = B
12-cent stamp = C
4 more 1-cent stamps than 8-cent stamps: A = 4 + B.
A = 4 + B
B = A - 4..........(1)
Twice as many 1-cent stamps as 12-cent stamps: A = 2C.
A = 2C
C = A/2...............(2)
Total number of stamps = 31
A + B + C = 31................(3)
Putting (1) and (2) in (3)
we get,
A + ( A - 4 ) + A / 2 = 31
A + A - 4 + A / 2 = 31
2A - 4 + A / 2 = 31
2A + A / 2 = 31 + 4
(4A + A) / 2 = 35
4A + A = 35 x 2
5A = 70
A = 70 / 5
A = 14
Thus the number of 1-cent stamps is 14.
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