To subtract, take ones from ones and tens from tens. Sometimes there aren’t enough ones. Then you break a ten into 10 ones (Trading 10 Ones for 1 Ten).
Por qué funciona
52 is 5 tens and 2 ones. You can’t take 7 ones away from 2 ones. Break one ten into 10 ones: now 52 is 4 tens and 12 ones. It is still 52, but now there are enough ones to take 7 away.
Ejemplo resuelto: No break
Find $68 - 23$.
- Ones: $8 - 3 = 5$.
- Tens: $6 - 2 = 4$.
- So $68 - 23 = 45$.
Respuesta: $45$
Ejemplo resuelto: Break a ten
Find $52 - 27$.
- Ones: 2 is less than 7. Break a ten: 4 tens and 12 ones.
- Ones: $12 - 7 = 5$.
- Tens: $4 - 2 = 2$.
- So $52 - 27 = 25$. Check: $25 + 27 = 52$.
Respuesta: $25$
Ejemplo resuelto: Count up
Find $52 - 27$ another way.
- Start at 27. Jump 3 to get to 30.
- Jump 20 to 50, then 2 to 52.
- $3 + 20 + 2 = 25$.
Respuesta: $25$
Idea clave
Subtract the ones, then the tens. If there are not enough ones, break a ten into 10 ones first. Check by adding your answer to the number you took away.
Error común
Taking the smaller digit from the bigger one in each place: for $52 - 27$, doing $7 - 2 = 5$ and $5 - 2 = 3$ to get 35. But you must take 7 away from 2, so break a ten. Check: $35 + 27 = 62$, not 52, so 35 can’t be right.
Grown-up tip: Have your child check every subtraction by adding. It catches most mistakes.