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(1) Consider the disk of the diameter 4 cm.
Calculate its volume = cubic centimeters.
Similarly, calculate the volume of the central hole of 2 cm in diameter.
From , subtract the volume of the central hole, You will obtain the volume of the solid metal.
(2) Do the same for two other disks and find the solid metal volumes and for that disks.
(3) Add , and to get the total solid metal volume.
(4) Multiply the total solid metal volume by the density to get the mass of the pulley.
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Having these instructions, you just know what to do.
The rest is on you.