To add two-digit numbers, add ones to ones and tens to tens. When the ones make 10 or more, trade them for a new ten, just like in Trading 10 Ones for 1 Ten.
Por qué funciona
Adding means putting all the blocks together. 8 ones and 7 ones make 15 ones, too many for one digit. Trade 10 of them for a new ten: now there is 1 more ten and 5 ones. The new ten must be counted with the other tens.
Ejemplo resuelto: No trade
Find $34 + 25$.
- Ones: $4 + 5 = 9$.
- Tens: $3 + 2 = 5$ tens.
- So $34 + 25 = 59$.
Respuesta: $59$
Ejemplo resuelto: With a trade
Find $38 + 47$.
- Ones: $8 + 7 = 15$. That is 1 ten and 5 ones. Write 5 and keep the new ten.
- Tens: $3 + 4 + 1 = 8$ tens.
- So $38 + 47 = 85$.
Respuesta: $85$
Ejemplo resuelto: Make a ten first
Find $38 + 47$ another way.
- Move 2 from 47 to 38: $38 + 2 = 40$ and $47 - 2 = 45$.
- $40 + 45 = 85$.
Respuesta: $85$
Idea clave
Add the ones, then the tens. If the ones make 10 or more, trade 10 ones for a new ten and add it to the tens.
Error común
Forgetting the new ten: $38 + 47 = 75$. The 15 ones held a hidden ten. Check by estimating: 38 is close to 40 and 47 is close to 50, so the sum is close to 90, not 75.
Grown-up tip: Ask your child where the little “1” comes from. If they can say “it’s the new ten,” they understand regrouping.