The cost of the mixture is $69 *$1 = $69. If all 69 pounds were peanuts, the cost would be 69*$0.99 = $68.31, which is $0.69 less. Each pound of walnuts used instead of peanuts adds $0.03 to the cost of the mixture, so there were $0.69/$0.03 = 23 pounds of walnuts.
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Let w represent the number of pounds of walnuts in the mixture. Then 69-w is the number of pounds of peanuts. The cost of the mixture will be
1.02w + 0.99(69-w) = 1.00*69 dollars
0.03w + 68.31 = 69.00
w = (69.00 - 68.31)/0.03 = 0.69/0.03 = 23
