Question 131069
"A cabbage and a cauliflower cost $3 in total, 
a cauliflower and a pumpkin cost $4 in total 
and a pumpkin and a cabbage cost $5 in total. 
Find the total cost of one of each vegetable."
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This is one way to work the problem; I'm sure there are others.
Let the cauliflower cost be "a"
Let the pumpkin cost be "b"
Let the cabbage cost be "c"
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Three equations
a  + 0b  + c  = 3
a  +  b  + 0c = 4
0a +  b  + c  = 5
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Subtract the 1st equation from the 2nd to get:
a  + 0b  +  c  = 3
0  +  b  -  c  = 1
0  +  b  +  c  = 5
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Subtract the 2nd equation from the 3rd to get:

a  + 0b  +  c  = 3
0  +  b  -  c  = 1
0  +  0  + 2c  = 4
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Now: 
1st: solve for c and get c = 2 (# of cabbages is 2)
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2nd: substitute into the 2nd equation to get:
b-2 = 1
b = 3 (# of pumpkins is 3)
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3rd: substitute into the 1st equation to get:
a + 0 + 2 = 3
a = 1 (# of cauliflower is 1)
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Good wishes on your studies.
Cheers,
Stan H.