The rules for determining the central number is different for each of the three puzzles below (A, B & C). Once you have determined the rule (orformula), you should be able to solve for the last square in each sequence...

A small hint if needed (but try solving first without the hint)... <span style="background-color: #FFFF00; color: #000000; font-weight: bold"><font color=yellow>The puzzle was made using Excel. The four small numbers on the outside of each bigger square were put in place first. Then a formula using those four numbers was written into the bigger square. The formula was then copied to each of the other big squares in the line. Except, the last square and that's for you to solve. So, if nothing is clicking, you might try using Excel.</font color=yellow></span hi>