If we assume the width of the frame is X. Then the one dimension of the framed picture will be 2X + 20 and the other dimension of the picture will be 2X + 34. We know the total area of the framed picture should be 640 in^2. Therefore:
(2X + 10) * (2X + 34) = 640
If you multiply this out you get:
4X^2 + 88X + 340 = 640
Subtract 640 from both sides:
4X^2 + 88X - 300 = 0
Divide both sides by 4:
X^2 + 22X - 75 = 0
Use the quadratic equation to solve for X, with A=1, B=22, C=75
X = [-22 +/- sqr(22^2-4*(-75)] /2
This reduces to:
(-22 +/- 48) / 2
The negative solution doesn't make sense in this case. Therefore: X = (-22 + 28) /2 = 6/3 = 3
Therefore the width of the frame should be 3 inches.
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