Each additional square (after the first) requires 3 more toothpicks (2 horizontal and 1 vertical), so 4 squares will require 13 toothpicks, 5 squares will require 16 toothpicks, 6 squares will require 19 toothpicks, 7 squares require 22 toothpicks, 8 squares require 25 toothpicks, 9 squares require 28 toothpicks, and 10 squares require a total of 31 toothpicks. Alternatively, notice that since each new square requires 3 more toothpicks AND we need 7 more squares (starting at the given figure), 10 squares will require 10 + (7 x 3), which equals 31 toothpicks.