3-18-33-48-63-78-93 6-16-26-36-46-56 11-18-25-32-39-46-53 14-22-30-38-46-54-62 Make all of your first numbers 0. So in the first one. 0 _ _ 45 _ _ 90 You subtract 3 from all numbers. The first gap is 0 too 45. So divide 45 by gaps including the last number. 0 is first number 45 is last. 45/3=15 0-15-30-45-60-75-90 Now add your 3 back in. 3-18-33-48-63-78-93