The diameter of the bottom of this Petri dish is 3 inches or 7.5 + 

 centimeters. 



The area of a circle is equal to the square of the radius multiplied 

 by TT or 22/7. 



i 1/2 in.=radius. i 1/2X1 1/2 = 2.25. 2.25X22/7 = 7.07 square 

 inches. 



3.75 cm. = radius. 3.75X3.75 = 14.06. 14.06X22/7=44.1 square 

 centimeters. 



Number of bacterial colonies in i sq. in. averages, approximately, 

 75. Number in 7.07 sq. in. = 530. 



Number of bacterial colonies in i sq. cm. averages, approximately, 

 12. Number in 44.1 sq. cm. = 5 28. 



In a microscopic field, if the diameter were 8 small squares (1/20 

 mm.), the radius would be 4 small squares and the area of such a round 

 field would be 4X4 = 16X22/7 = 50+. Such a field would contain 50 

 small squares. 



