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I do not think that this problem assumes getting precisely correct number of firebricks.
I think that obtaining a reasonable estimation is totally enough.
To get the estimation, take the area of the circle and divide it by the area of one single firebrick.
The area of the circle is = = = 1133.54 square inches.
The area of the firebrick is 9*4.5 = 40.5 square inches.
So the answer (the final estimation) is N = = ~ 28 firebricks. ANSWER
Another, more conservative estimation is to increase the radius of the circle by half the diagonal of the firebrick
and re-estimate with this new data.
By knowing the idea, you can make all necessary calculations on your own.
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To wide your horizon, you may want to look some sources from the Internet
https://math.stackexchange.com/questions/2984061/cover-a-circle-with-squares