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Question
A chart related to the problem only indicates for a 1-2-3 mixture with 7 bags of cement, 14 cu. ft. of sand and 21 cu. ft. of gravel are required for 1 cu. yard of concrete. Maybe that will help.  Thanks again.

Answer
Let θ be the base angle of the post, then tanθ = 96.
https://www.flickr.com/photos/dwread/15091183430/ (drawing not to scale)

Now I'll switch to working in feet. (I'll use fractions to avoid rounding errors.)
The post is 8 feet high, its base is (5/12) ft by (5/12) ft, and its top is ft by ft.

The post is the bottom section of a square pyramid.
https://www.flickr.com/photos/dwread/15092592898/ (drawing not to scale)
height of pyramid = (5/24)tanθ = 20 ft
volume of pyramid = (⅓)bh = (⅓)(5/12)20 ≅ 125/108 ft

The top section of the pyramid is a smaller pyramid. It is 20-8 = 12 ft high and its base is ft by ft.
volume of smaller pyramid = (⅓)bh = (⅓)()12 = ft
volume of post = 125/108 - = 49/54 ft

7 bags of cement are required for 1 yd = 27 ft of concrete.
8 bags   27 ft/(7 bags) = 216/7 ft
Using 8 bags of cement, you can make 216/7 ft of concrete.

216/7 ft 1 post/(49/54 ft) ≅ 34.00583 posts
You can make 34 posts, which will require 34 posts 49/54 ft/post = 833/27 ft of concrete.

Each cubic foot of concrete contains 7/27 bag of cement, 14/27 ft of sand, and 21/27 = 7/9 ft of gravel. So, to make 34 posts you will use
 833/27 ft concrete 7/27 bag cement/(1 ft concrete) ≅ 7.999 bag cement
 833/27 ft concrete 14/27 ft sand/(1 ft concrete) ≅ 15.997 ft sand
 833/27 ft concrete 7/9 ft sand/(1 ft concrete) ≅ 23.996 ft gravel
You can probably round that to 8 bags of cement, 16 cubic feet of sand, and 24 cubic feet of gravel.

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