f = flour and s = sugar
12s + 15f = 9.30
4s + 10f = 4.60.....multiply by -3
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12s + 15f = 9.30
-12s - 30f = - 13.80(result of multiplying by -3)
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-15f = - 4.50
f = -4.50 / -15
f = 0.30 <=== flour costs 30 cents per lb
4s + 10f = 4.60
4s + 10(0.30) = 4.60
4s + 3 = 4.60
4s = 4.60 - 3
4s = 1.60
s = 1.60/4
s = 0.40 <== sugar costs 40 cents per lb