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 * or 22/7. 



i 1/2 in. =radius. 11/2x11/2=2.25. 2.25 x 22/7 = 7.07 square 

 inches. 



3.75 cm. =radius. 3.75x3.75 = 14.06. 14.06 x 22/7 = 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, approxi- 

 mately, 12. Number in 44.1 sq. cm. = 528. 



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 4 x 4 = 16 x 22/7 = 50 + . Such a field would 

 contain 50 small squares. 



