Multiples of ten and one hundred are much easier to add together! Multiples of ten are easier to manage, but using hundreds can be helpful for larger numbers.  Always round up because it’s easier to keep track of how much you’re adding to your original numbers as you round. Here’s an example problem: 423+678. You’d round it like this: 430 + 680 or 500+700. You’ll need to take away the numbers you added when rounding. You can count off these numbers or simply add them in your head. Hold onto this number. In the problem listed above, you’ll count out 7 places for rounding 423 to 430 and 2 places for rounding 678 to 680. Add 7+2=9. Since you rounded, it’s easier to calculate this total in your head. However, this isn’t your final answer! For the problem above, you’d add 430+680=1110 You can use simple subtraction or count backwards from the sum of your rounded numbers. This will give you your final total! To solve the problem above, you’d subtract or count back 1110-9 =1101. When you’re adding several numbers together, you may find it easier to pair numbers together to create round numbers that end in “5” or “0,” then add those round numbers together. For example, let's say you're adding 1+8+2+7+3. You could add 8+2 = 10 and 7+3=10. That leaves you with 1+10+10=21. Break the number apart into just tens and ones. For example, 42+35+17 would become two problems: 40+30+10 and 2+5+7. It’s easy to add 40+30+10=80. Next, you’d just need to count 2+5+7=14. Finally, add your two sums together to get a final sum of 80+14=94. You can also do this with numbers in the hundreds. You can break the number down into just the hundreds, just the tens, and just the ones. However, using one of the other techniques may be easier when adding big numbers.
Round all of your numbers to multiples of ten or one hundred. Count out how many numbers you added when you rounded. Add the two rounded numbers together in your head. Subtract the number you added when rounding. Add the numbers in sets, then add the set totals together, as an alternative. Add the numbers by places if that’s easier for you.