The volume of a cylinder (this formula is given to you in the beginning of every math section) is:
![image](https://i0.wp.com/78.media.tumblr.com/1f554733e96e489142949acd08f431d1/tumblr_inline_pb6qzrLBn71qio24w_540.png?w=852&ssl=1)
So if the volume of a particular cylinder is 100 cm^3, you know this:
![image](https://i0.wp.com/78.media.tumblr.com/3ac472c7d3dbc4c59520e01a5c456880/tumblr_inline_pb6r0l7v451qio24w_540.png?w=852&ssl=1)
Now apply what the question says to what you know. Cut the radius in half and double the height:
![image](https://i0.wp.com/78.media.tumblr.com/5b2030584a22204c1411ae204a8134fd/tumblr_inline_pb6r7m82Ij1qio24w_540.png?w=852&ssl=1)
But wait—we can substitute! We already know the top half of that fraction equals 100:
![image](https://i0.wp.com/78.media.tumblr.com/5a48c56bdcac5efc6204d3d6f9825998/tumblr_inline_pb6r9ygevu1qio24w_540.png?w=852&ssl=1)
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