let 'x' be the number of grams at 1peso
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since he is to sell to make the average price point be 0.85pesos/gram then,
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[15 (0.75) +(x)(1)]÷(15+x)] = 0.85
we add the total sales be divided by the total weight as the price is per gram. Since we say that the 1st totals to 15 grams and that the other as 'x' then we divide the total sales by 15+x
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cross multiply 15+x to the right side of the equation
15 (0.75) +x (1) = 0.85 (15+x)
11.25 + x = 12.75 + 0.85x
transpose 0.85x to the left side of the equation and 11.25 to the right side of the equation
x - 0.85x = 12.75-11.25
0.15x = 1.5
x = 10
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therefore you'll need 10 grams of nuts at one peso