Lisa and Alana from Sudbourne School, Lambeth used words to
  explain how they worked out the next number: <br />
  "The answer to what is the next number is 163 because the
  difference between 3 and 7 is 4 and the next difference (between
  7 and 19) is 12 which is 3x4. The difference after that (between
  19 and 55) is 36 which is 12 (the last difference)times 3 (the
  first number, also the number timsed by 4 to get 12.) So we just
  added 108 to 55 which equals 163". <br />
  Sinan from IRMAK Primary School, Istanbul, Turkey says much the
  same thing with numbers:<br />
  <img width="521" height="158" src=
  "/content/00/10/penta3/Image1.gif" alt="1" /><br />
  Sarah from Annesley College, Adelaide South Australia explained
  it a slightly different way: "I figured it out by working out how
  much the difference is between the four numbers. Then I found
  that all the numbers between are all factors of three. So, I
  times each of the between numbers by three. When I got to 55
  times three I got 108. Then I added 55 and 108 and I got 163.
  This was the answer I got."<br />
  <br />
  Shivana from Henry Park Primary School, Singapore has another way
  of writing it: The answer is 163. n x 3 - 2 (n being the previous
  number).<br />
  Example 19x3-2=55<br />
  <br />
  <br />