Question

A biologist used a microscope to measure two different kinds of cells. Cell A was 12 micrometers in diameter, and cell B was 20 micrometers in diameter. She then took a photograph of the two cells. In the photograph, cell A measured 15 cm in diameter. What was the diameter of cell B in the photograph?

Answer 1

The concept to be used in order to determine the diameter of the second cell in the photograph is that of the ratio and proportion. The ratio of the first set of measurements should be proportional (equal) to the ratio of the second set. 

Ratio of first set of diameters,
   r = diameter of Cell A / diameter of Cell B
   r = 12 micrometers/20 micrometers
   r = 3/5

If we let D be the diameter of the cell B in photographs,
  r = 3/5 = 15 cm/x cm
The value of x from the equation is 25. Hence, the diameter of cell B is 25 cm.