The sign says miles. Your friend thinks in kilometers. The problem hands you the rate. You switch.
A sticker is 5 inches wide. A road sign thinks in miles. A conversion switches one unit to another that measures the same thing. Inches and centimeters both measure length. Miles and kilometers both measure a road.
The question gives a conversion, such as 1 inch = 2.54 centimeters. This tells you how much of one unit equals another. The symbol ≈ means “approximately equal to,” or “about equal to.” Use the conversion given in the question.
Use the conversion to decide whether to multiply or divide. For 1 inch = 2.54 centimeters, multiply inches by 2.54 to find centimeters; divide centimeters by 2.54 to find inches. You can also write a proportion—two equal ratios: 1 inch / 2.54 cm = 5 inches / x cm. Here x is the unknown length in centimeters.
Then check the size. Five inches should come out a little over 12 centimeters, not half a centimeter. If the number looks backwards, you multiplied when you should have divided.
Try it. Then open the steps.
Example 1 of 5
A ruler on the desk
A sticker is 5 inches wide. Use 1 inch = 2.54 centimeters. How many centimeters wide is it?
| Inches | Centimeters |
|---|---|
| 1 | 2.54 |
| 5 | ? |
Answer: 12.7 cm
Example 2 of 5
Highway, two numbers on one sign
A trip is 10 miles. Use 1 mile ≈ 1.6 kilometers. How many kilometers?
Answer: 16 km
Example 3 of 5
A bag of rice is 4 kilograms. Use 1 kilogram ≈ 2.2 pounds. How many pounds?
Answer: 8.8 lb
Example 4 of 5
A recipe from a metric cookbook
A recipe uses 2 liters of broth. Use 1 liter ≈ 1.06 quarts. How many quarts?
Answer: 2.12 qt
Example 5 of 5
A shelf is 3 feet 4 inches long. 1 foot = 12 inches, and 1 inch = 2.54 centimeters. How many centimeters long is the shelf?
Answer: 101.6 cm
Easy to miss: Multiplying when you should divide. 10 kilometers is not 10 × 1.6 miles.
Do this: A mile is longer than a kilometer. 10 km is fewer miles: 10 ÷ 1.6 = 6.25. Ask: should the new number be bigger or smaller?
Easy to miss: Grabbing the wrong rate, like 2.54 on a miles problem.
Do this: Copy the rate from the sentence. Use only that rate.
24 problems · Camp, Trail, Summit
