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5 ft · 12 in 1 ft = 60 in

We treat the unit words like factors and ‘divide out’ common units like we do common factors.

Make unit conversions.

  1. Multiply the measurement to be converted by 1 ; write 1 as a fraction relating the units given and the units needed.
  2. Multiply.
  3. Simplify the fraction, performing the indicated operations and removing the common units.

Mary Anne is 66 inches tall. What is her height in feet?

Solution

Convert 66 inches into feet.
Multiply the measurement to be converted by 1. 66 inches · 1
Write 1 as a fraction relating the units given and the units needed. 66 inches · 1 foot 12 inches
Multiply. 66 inches · 1 foot 12 inches
Simplify the fraction. 66 inches · 1 foot 12 inches
66 feet 12
5.5 feet

Notice that the when we simplified the fraction, we first divided out the inches.

Mary Anne is 5.5 feet tall.

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Lexie is 30 inches tall. Convert her height to feet.

2.5 feet

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Rene bought a hose that is 18 yards long. Convert the length to feet.

54 feet

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When we use the Identity Property of Multiplication to convert units, we need to make sure the units we want to change from will divide out. Usually this means we want the conversion fraction to have those units in the denominator.

Ndula, an elephant at the San Diego Safari Park, weighs almost 3.2 tons. Convert her weight to pounds.

A photograph of an adult elephant.
(credit: Guldo Da Rozze, Flickr)

Solution

We will convert 3.2 tons into pounds, using the equivalencies in [link] . We will use the Identity Property of Multiplication, writing 1 as the fraction 2000 pounds 1 ton .

3.2 tons
Multiply the measurement to be converted by 1. 3.2 tons · 1
Write 1 as a fraction relating tons and pounds. 3.2 tons · 2000 lbs 1 ton
Simplify. 3.2 tons · 2000 lbs 1 ton
Multiply. 6400 lbs
Ndula weighs almost 6,400 pounds.
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Arnold’s SUV weighs about 4.3 tons. Convert the weight to pounds.

8600 pounds

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A cruise ship weighs 51,000 tons. Convert the weight to pounds.

102,000,000 pounds

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Sometimes to convert from one unit to another, we may need to use several other units in between, so we will need to multiply several fractions.

Juliet is going with her family to their summer home. She will be away for 9 weeks. Convert the time to minutes.

Solution

To convert weeks into minutes, we will convert weeks to days, days to hours, and then hours to minutes. To do this, we will multiply by conversion factors of 1 .

9 weeks
Write 1 as 7 days 1 week , 24 hours 1 day , 60 minutes 1 hour . .
Cancel common units. .
Multiply. 9 · 7 · 24 · 60 min 1 · 1 · 1 · 1 = 90 , 720 min
Juliet will be away for 90,720 minutes.

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The distance between Earth and the moon is about 250,000 miles. Convert this length to yards.

440,000,000 yards

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A team of astronauts spends 15 weeks in space. Convert the time to minutes.

151,200 minutes

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How many fluid ounces are in 1 gallon of milk?

A photograph of a milk display in a grocery store.
(credit: www.bluewaikiki.com, Flickr)

Solution

Use conversion factors to get the right units: convert gallons to quarts, quarts to pints, pints to cups, and cups to fluid ounces.

1 gallon
Multiply the measurement to be converted by 1. 1 gal 1 · 4 qt 1 gal · 2 pt 1 qt · 2 C 1 pt · 8 fl oz 1 C
Simplify. 1 gal 1 · 4 qt 1 gal · 2 pt 1 qt · 2 C 1 pt · 8 fl oz 1 C
Multiply. 1 · 4 · 2 · 2 · 8 fl oz 1 · 1 · 1 · 1 · 1
Simplify. 128 fluid ounces
There are 128 fluid ounces in a gallon.
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Source:  OpenStax, Prealgebra. OpenStax CNX. Jul 15, 2016 Download for free at http://legacy.cnx.org/content/col11756/1.9
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