Write what kind or how many for each adjective.

E=number of eight cent stamps
t=number of ten cent stamps bought
w=number of 2 cent stamps bought

number of eight=number of ten
e=t

as many 2 cent as the other 2 combined
e+t=w or
2e=w=2t

total she bouthgt 4.40
4.40=440 cents
8e+10t+2w=440
solve for t or e, I solve for t
8t+10t+2(2t)=440
18t+4t=440
22t=440
divide 22
t=20

t=e
20=e
w=2e
w=2(20)
w=40

answer is
20 eight cent
20 ten cent
40 two cent

Write it out as a set of equation:
Let x be number of 8 cent stamps, y be 10 cent stamps, and z be 2 cent stamps.
x=y
z=x+y
8x+10y+2z=440
Lets first solve for x:
from x=y and z=2x(from first equation) the last equation is
8x+10x+4x=440
22x=440
x=20
know that x=20, you also know that y=20 as well, since z=x+y, z=40.
So 20 8-cent stamps, 20 10-cent stamps, and 40 2-cent stamps.

X=number of 8-cent stamp=number of 10-cent stamp
y=number of 2-cent stamp.
$4.40=400 cents

She bought the same number of 8.cent stamp and 10 -cent stamps, she also bought as many 2-cent stamps as both, therefore:
x+x=y  ⇒2x=y
We can suggest this system of equations:

2x=y
8x+10x+2y=440

We can solve this system of equation by substitution method.

8x+10x+2(2x)=440
8x+10x+4x=440
22x=440
x=440/22
x=20

y=2x
y=2(20)=40

She bought 20 stamps of 8 cents, 20 stamps of 10 cents, and 40 stamps of 2 cents