I have a tree 8in at one end 20in at the other and 16ft iong how many cubic yard are there

I think this is about the volume of wood in the log.

First we need the total length if it tapered to a point. In a length of 16 x 12 = 192" it tapered down by 20 - 8 = 12" , which is 1" for each
192 / 12 = 16" of length. So if the tree tapered right out to a point, the extra would be 8 x 16 = 128" length, for a total of 128 + 192 = 320" length.

Now the volume of a cylindrical cone is 1/3 * Pi * r^2 * h, so the whole log including the imaginary point is 1 /3 * Pi * 10^2 * 320 = 33510.3 cu in.

The volume of just the imaginary bit is 1 /3 * Pi * 4^2 * 128 = 2144.7. The solid part has a volume then of 33510.3 - 2144.7 = 31365.6 cu in.

There are 36 x 36 x 36 = 46656 cu in per cu yd, so the log has a volume of 31365.6 / 46656 = 0.67 cu yd, or 2/3 cu yds.

Study this and make sure you understand it. Don't merely copy it because you will learn nothing and will not know how to deal with the next problem.
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Http://www.wikihow.com/Find-the-Surface-Area-of-Cones
How to Find the Surface Area of Cones This is the link to the best description on how to work out the surface area of a cone. In your case you need to work out the total surface area of the cone and then subtract the smaller cone, (the invisible one) on the top of the tree.
Just me, but it would be easier to convert the equation to metric, get the equivalent square meters and then convert to square yards using Google help. I never learnt imperial measurements. Hope this helps. Cheers Bob