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.
Why it works
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.
Key idea
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.
Worked example: Make a ten
Find $2 \times 7 \times 5$.
- Group the 2 and the 5: $2 \times 5 = 10$.
- $10 \times 7 = 70$.
Answer: $70$
Worked example: 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.
Answer: $24$
Common mistake
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.