We suggest that you start with the next problems:

LOGICAL PROBLEMS 

You can also watch on Youtube Logical Problems for 5 graders

CALCULATE QUICKLY!

The next set of problems deals with natural numbers

NATURAL NUMBERS

EVEN NUMBERS

PRIME NUMBERS

ARITHMETIC PUZZLES

The last presentation does not include answers, but all problems could be solved with 

PIGEONHOLE PRINCIPLE

Or watch on Youtube The Pigeonhole Principle