Adding a multiple of ten to any two-digit number changes only the tens digit — the ones digit stays the same. For example, 34 + 20 = 54 because 3 tens + 2 tens = 5 tens, and the 4 ones are untouched. This mental math skill flows directly from place value understanding and previews column addition.
Use base-ten blocks: add ten-rods without disturbing the unit cubes. Show on a hundred chart that adding 10 moves one row down while staying in the same column (same ones digit). Connect to skip counting by 10s from a non-round starting number.
You've already learned place value — that a two-digit number is made of tens and ones, and that the digits in each place carry independent values. You've also practiced addition within 10. Adding tens to a two-digit number is the first time you use both of those things at once: it's a test of whether you really understand that the tens and ones places work independently.
Here is the key insight: when you add a multiple of ten (like 20, 30, or 40) to a two-digit number, only the tens digit changes. The ones digit stays exactly the same. Try it: 34 + 20. The 34 has 3 tens and 4 ones. The 20 has 2 tens and 0 ones. You're adding 3 tens and 2 tens, which gives 5 tens. The 4 ones haven't been touched. So the answer is 54 — same ones digit, new tens digit. This works every time because adding a round ten contributes zero new ones.
The hundred chart makes this visible. Find 34 on the chart. Adding 10 moves you one row down — to 44. Adding 10 again moves you to 54. Notice that you've stayed in the same column the entire time. The column is determined by the ones digit, which never changed. This visual confirms what place value tells you: tens and ones are separate tracks, and adding tens only moves you along the tens track.
The most common mistake is treating both digits as if they interact — thinking 34 + 20 means "3 + 2 = 5 and 4 + 2 = 6, so 56." This confusion disappears if you return to place value: the 2 in 20 is 2 tens, not a plain 2. It can only be added to the tens digit of 34, not to the ones digit. Think in place value, not in digits. Whenever you see a number like 20 or 30 in an addition problem, immediately translate it: "that's 2 tens" or "that's 3 tens" — then you'll naturally add it only to the tens column and leave the ones digit untouched.