This is a photo of a puzzle my facebook friend took, supposedly from a 5th grader's school book. For both puzzles the bottom row with two patterns is missing. The puzzle "a" is fairly simple one, but I don't think many 5th graders (so 10-11 years olds) could get the "b" right (or then the real solution is simpler than I think!).
What's the solution to the puzzle B?
Bonus: can you get a 10-11 year old to solve this? :-)