A foot is 12 inches, an hour is 60 minutes, and a kilogram is 1,000 grams. Changing a big unit into a small one is multiplication, the same kind you did in multiplying by 10, 100 and 1,000.
Idea clave
To change a larger unit into a smaller unit, multiply by how many small units are in one large unit: $3$ feet $= 3 \times 12 = 36$ inches; $2$ kg $= 2 \times 1{,}000 = 2{,}000$ g. The number gets bigger because the unit got smaller.
Por qué funciona
Each foot becomes 12 inches, so 3 feet become 3 groups of 12 inches. Smaller units fit more times into the same length, so you need more of them. A table makes the pattern easy to see: each row adds one more group.
| Large unit | Small unit | How many in one |
|---|---|---|
| 1 foot | inches | 12 |
| 1 yard | feet | 3 |
| 1 meter | centimeters | 100 |
| 1 kilometer | meters | 1,000 |
| 1 kilogram | grams | 1,000 |
| 1 pound | ounces | 16 |
| 1 liter | milliliters | 1,000 |
| 1 hour | minutes | 60 |
| 1 minute | seconds | 60 |
Ejemplo resuelto: One unit
How many inches are in 5 feet?
- 1 foot = 12 inches.
- $5 \times 12 = 60$.
Respuesta: 60 inches
Ejemplo resuelto: Mixed units
How many minutes are in 2 hours 15 minutes?
- 2 hours = $2 \times 60 = 120$ minutes.
- Add the 15 minutes: $120 + 15 = 135$.
Respuesta: 135 minutes
Error común
Dividing because "the unit is getting smaller," so 3 feet becomes $3 \div 12$. Ask: will there be more small units or fewer? More, so multiply. Check: 36 inches should be about the length of 3 rulers.