Now we learn the facts so they come fast. Start with three easy families. You already know how to count by 5s and 10s, and with turn-around facts each fact does double duty.
Por qué funciona
Times 2 is a double: $2 \times 8 = 8 + 8$. Times 10 makes tens: $10 \times 6$ is 6 tens, 60. Times 5 is half of times 10, because 5 is half of 10: $10 \times 7 = 70$, so $5 \times 7 = 35$.
Idea clave
2s: double the number. 10s: that many tens. 5s: half of the 10s fact, or count by 5s.
Ejemplo resuelto: A double
Find $2 \times 8$.
- Double 8: $8 + 8 = 16$.
Respuesta: $16$
Ejemplo resuelto: Half of ten
Find $5 \times 7$.
- $10 \times 7 = 70$.
- 5 sevens is half of 10 sevens: half of 70 is 35.
Respuesta: $35$
Ejemplo resuelto: Tens
Find $10 \times 9$.
- 10 groups of 9 is the same as 9 groups of 10: 9 tens.
- 9 tens is 90.
Respuesta: $90$
Error común
Losing count while skip counting. For $5 \times 8$, keep track of how many 5s you have said, on your fingers. Or use half of $10 \times 8 = 80$, which is 40. Two ways that agree make a strong check.