Sometimes you multiply three numbers, like 2 boxes, each with 5 bags, each with 7 marbles. You have the facts; now the question is which two to multiply first.
Por qué funciona
The block has the same cubes however you count them. Count by layers: $(2 \times 5) \times 3$. Count by the slices standing up: $2 \times (5 \times 3)$. Same block, same 30 cubes. So you may group the factors any way you like, and pick the easiest.
Idea clave
When you multiply three numbers, you can multiply any two first: $(2 \times 5) \times 7 = 2 \times (5 \times 7)$. Look for a pair that makes 10.
Ejemplo resuelto: Make a ten
Find $2 \times 7 \times 5$.
- Group the 2 and the 5: $2 \times 5 = 10$.
- $10 \times 7 = 70$.
Respuesta: $70$
Ejemplo resuelto: Another grouping
Find $3 \times 2 \times 4$.
- $3 \times 2 = 6$, then $6 \times 4 = 24$.
- Or $2 \times 4 = 8$, then $3 \times 8 = 24$. Same answer.
Respuesta: $24$
Error común
Adding when you meant to group. $(2 \times 3) \times 4$ is 24, not $2 \times 3 + 4 = 10$. The parentheses only show which two to multiply first; every number is still multiplied.