Hi there,

Unfortunately, we don't have too many worksheets with that particular type of math problem. We do have a number order worksheet (first link below) that includes a few number riddles similar to your example, but as it's a first grade worksheet it may not be challenging enough for your third grader.

We also have a math riddle game called Give Me a Clue that uses playing cards (see second link below). It's a simple number guessing game in which one player gives clues about two of the cards in his hand using math operations while the other player tries to guess the numbers. You could extend the game to include place value and odd/even clues as well to help your son in those areas. The great thing about this game is that your son will not only practice guessing the numbers, he'll also have a chance to come up with his own clues, which I think will help him better understand how the riddles work as well as how to solve them. And all you need is a deck of cards to start playing!

Finally, you could also give your son worksheets on place value, rounding, and even/odd numbers, which are all concepts he needs to understand in order to solve the problem you've given above. I've linked to a selection of 3rd grade worksheets (3rd link below) that should help your son tackle these concepts. The second worksheet in that list, called "Math Reveiw: Test Your Skills", is one you might want to check out first, as it covers all of these skill areas and also includes inequalities and missing operation problems.

Good luck, and hope this helps!

First of all 7+8=15
So 78 is the answer, which is also even (ones place)
78 rounds to 80, because the 8 is 5 or more, add one more to the 10's place getting to the number 80.