Asquare pyramid that is 20 meters tall is cleanly sliced through by a laser, 1.5 meters above its base (this leaves a shape similar to a prism). if its base has an area of 6 square meters, which is the best estimate of the volume of the remnant, which is 1.5 meters tall?

I believe your answer is 9. The formula for volume is: V=bh (or V=lwh). We already have the base so we multiply that by the height, which is 1.5. Plug the numbers in to get: 6 x 1.5 = 9.
I believe the best estimate of the volume of the remnant is 9 meters³.

Hope this helps,
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