SOLUTION: An inventory manager orders 6 batteries, 5 cases of spark plugs, and 2 dozen pairs of wiper blades, she will pay $830. If she orders 3 batteries, 7 cases of spark plugs, and 4 doz

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Question 279308: An inventory manager orders 6 batteries, 5 cases of spark plugs, and 2 dozen pairs of wiper blades, she will pay $830. If she orders 3 batteries, 7 cases of spark plugs, and 4 dozen wiper blades, she will pay $820. If the batteries are $22 less than twice the price of a dozen wiper blades, what is the cost of each item in her order?
Answer by josmiceli(19441) About Me  (Show Source):
You can put this solution on YOUR website!
Let a = cost of a battery
Let b = cost of a case of spark plugs
Let c = cost of a dozen pair of wiper blades
given:
(1) 6a+%2B+5b+%2B+2c+=+830
(2) 3a+%2B+7b+%2B+4c+=+820
(3) a+=+2c+-+22
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This is 3 equations and 3 unknowns, so it's solvable
Multiply both sides of (2) by 2 and subtract
(1) from (2)
(2) 6a+%2B+14b+%2B+8c+=+1640
(1) -6a+-+5b+-+2c+=+830
9b+%2B+6c+=+810
9b+=+810+-+6c
b+=+90+-+%282%2F3%29%2Ac
By substitution:
(2) 3a+%2B+7b+%2B+4c+=+820
3%2A%282c+-+22%29+%2B+7%2A%2890+-+%282%2F3%29%2Ac%29+%2B+4c+=+820
6c+-+66+%2B+630+-+%2814%2F3%29%2Ac+%2B+4c+=+820
%2816%2F3%29%2Ac+=+820+-+630+%2B+66
%2816%2F3%29%2Ac+=+256
16c+=+768
c+=+48
And, from (3),
a+=+2c+-+22
a+=+2%2A48+-+22
a+=+74
And, from (1)
(1) 6a+%2B+5b+%2B+2c+=+830
6%2A74+%2B+5b+%2B+2%2A48+=+830
444+%2B+5b+%2B+96++=+830
5b+=+830+-+444+-+96
5b+=+290
b+=+58
$74 = cost of a battery
$58 = cost of a case of spark plugs
$48 = cost of a dozen pair of wiper blades
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check answer:
(2) 3a+%2B+7b+%2B+4c+=+820
3%2A74+%2B+7%2A58+%2B+4%2A48+=+820
222+%2B+406+%2B+192+=+820
820+=+820
OK