Adding Three or Four Two-Digit Numbers

When you have to add three or four two-digit numbers together, the problem might look a little long! But don't worry—by using a few smart strategies, you can break the problem down and make the addition much easier.

Strategy 1: Look for Friendly Pairs

Instead of just adding straight from left to right, look for pairs of numbers that end in digits that add up to $10$ (like $1+9$, $2+8$, or $5+5$). When you add these pairs together, they make a "round" number ending in zero (like $20, 30,$ or $40$), which is much easier to work with.

Example 1: Adding Three Numbers

Let's solve: $25 + 37 + 15 = ?$

1. Find a friendly pair: Look at the ones digits: $5$, $7$, and $5$. Notice that $5 + 5 = 10$.
2. Group those numbers together: $25 + 15 = 40$
3. Add the remaining number: Now, take your round number ($40$) and add the $37$ to it. $40 + 37 = 77$

So, $25 + 37 + 15 = 77$.

Example 2: Adding Four Numbers

Let's solve a bigger problem: $12 + 28 + 33 + 17 = ?$

1. Find friendly pairs: Look at the ones digits ($2, 8, 3, 7$) to find pairs that make $10$.
   • Pair 1: $12$ and $28$. Since $2 + 8 = 10$, we know $12 + 28 = 40$.
   • Pair 2: $33$ and $17$. Since $3 + 7 = 10$, we know $33 + 17 = 50$.
2. Add the new totals together: Now you just have to add your two round numbers. $40 + 50 = 90$

So, $12 + 28 + 33 + 17 = 90$.

Strategy 2: Stacking the Numbers

If you can't find friendly pairs easily, you can always stack the numbers vertically!

1. Line up all the tens and ones in neat columns.
2. Add all the numbers in the ones column first.
3. If the ones add up to $10$ or more, write down the ones digit and carry over the extra tens to the top of the tens column.
4. Add all the numbers in the tens column (don't forget the number you carried over!).

Both strategies work perfectly. Finding friendly pairs is a great mental math trick, while stacking is a reliable way to solve any addition problem you face!