Third graders will love this addition puzzle. It's more complicated than it looks but challenges kids to figure out the missing numbers, complete the sums, and do some mental math. Doing three-digit addition mentally involves adding numbers from the right to the left, starting with the ones place, then tens, and finally hundreds. For instance, when adding 345 + 678, first add 5+8 to get 13 and carry the 1 to the tens place. Then, add 4+7+1 (carried over) to get 12, carry the 1 to the hundreds place. Lastly, add 3+6+1 (carried over) to get 10. So, the answer is 1023. If the sum in any place is less than 10, no carrying is needed.