Answer: 24000 mm^3
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There are a few ways to do this.
One way is to slice horizontally to break up the L shape into two separate rectangular blocks. Refer to the diagram below to see what I mean.
The block on top has length 80-60 = 20 mm (horizontal), width 10 mm, and height 40 mm.
The volume of this block on top is L*W*H = 20*10*40 = 8000 mm^3, where mm^3 means "cubic millimeters"
The block down below has a length of 80 mm, width of 10 mm, and height of 60-40 = 20 mm (vertical). Its volume is L*W*H = 80*10*20 = 16000 mm^3
Add up the two volumes to get the overall volume: 8000+16000 = 24000 mm^3
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Another approach you could take is to do a vertical slice to form two separate blocks. You would follow similar steps mentioned in the section above to find the two volumes, and add up the smaller pieces.
Yet another way to find the answer is to find the area of the L shape, and then multiply that by the depth of 10 mm.