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Old Aug 9th 2017, 04:15 AM   #1
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Demsity-Please help

Density of a regular wooden block is 0,8 gr/cubic centimeters. The Mass of the
wooden block is 800 grams. We hollow the wooden block and put melted iron into the hole with a density of 7,8 g/cubic centimeters.The Total mass of the system becomes 1.5 kilograms. Find the volume of the hole(hollow)?

Answer is 100 but I can't find the answer.
I found the volume of the block as 1000 cc.
density=mass/volume
7.8=(1500-800)/V
I found the answer as 90 but the answer is 100. Please help.
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Old Aug 9th 2017, 05:19 AM   #2
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You left out the fact that once the hole is cut in the wood the volume of wood remaining is less than 1000 cm^3. Let V = the volume of the hole, then the mass of the remaining wood is (1000 cm^3 - V) x 0.8 g/cm^3. The mass of the iron is V x (7.8 g/cm^3). So the equation is:

(1000-V) x 0.7 + V x 7.8 = 1500

Solve for V.
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Old Aug 9th 2017, 10:36 AM   #3
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Many Thanks.
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