# Page:Popular Science Monthly Volume 13.djvu/485

1. Example: There are 5 brothers or sisters and 5 cousins whose portraits are available; the total period of desired exposure is 100 seconds. 5 ${\displaystyle \times }$ 4 ${\displaystyle +}$ 5 ${\displaystyle =}$ 25; 10025 ${\displaystyle =}$ 4; which gives 4 ${\displaystyle \times }$ 4 ${\displaystyle =}$ 16 seconds for each brother or sister, and 4 seconds for each cousin (5 x 16 ${\displaystyle +}$ 5 x 4 ${\displaystyle =}$ 100).