Reed uses 8 pieces of glass per square foot to make a stained glass window. how many pieces of glass will he need to make a window that is 864 square inches?

The most important information that needs to be kept in mind is the conversion from square foot to square inches. Other important information's are already given in the question.
1 square foot = 144 square inches
Now
Number of glasses required to cover 1 square foot = 8
Then
The number of glasses required to cover 144 square inches = 8
So
The number of glasses
required to cover 864 square inches = (8/144) * 864
                                                           = 8 * 6
                                                           = 48
So 48 pieces of glass are required to make a window that is 864 square inches. I hope the procedure is clear enough for you to understand.

Add up all the sides