My Fourth Grader had this for homework:
1562 _____ _____ _____ _____ _____ 1874
Fill in the missing numbers on the number line above, and write down the strategy as you would explain it to someone else.
(To be fair, this was the last problem. The first two made it possible for even a dullard like me to work out the rule. This was a test to see if I, erm, the student got it right.)
The solution, as I finally worked it out, is to find the difference, and divide that by n+1 where n is the count of missing numbers. That gives you the value to add to the first number and each succeeding number. So in the example,
1874 – 1562 = 312.
312 / 6 = 52.
1562 1614 1666 1718 1770 1832 1874
Struck me as pretty hard for a fourth grader, but that’s more about me than any actual fourth grader.