Teaching Notes 2.14: Writing and Solving Proportions
A common error in writing and solving proportions occurs when students set up the proportion
incorrectly. They then compare incorrect items or rates.- Explain that a proportion is a statement that two ratios are equal. In order to do this, the
units in the first term of each ratio must be the same. Likewise, the units in the second
term must also be the same. For example, if in the first ratio apples are compared to oranges,
apples must be compared to oranges in the second. If apples are compared to oranges in the
first and oranges are compared to apples in the second, the proportion is set up incorrectly. - Review the example on the worksheet with yourstudents. Note how the ratios are written.
Direct your students’ attention to the units and emphasize that each ratio is almonds to
ounces. To avoid mistakes in setting up a proportion, encourage your students to write the
units of the ratios first and then write the numbers. - Depending on the abilities of your students, you may find it helpful to review solving propor-
tions. You may also want to review rounding.
EXTRA HELP:
Always double-check that each ratio in the proportion is a comparison of the correct units.ANSWER KEY:
(1)900 words;60
1
=
x
15(2)$0.15;
20
2. 99
=
1
x(3)48;
4
96
=
x
1152
(4)26.35 minutes;8. 5
1
=
x
3. 1
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(Challenge)Yes. If Luis reads at a rate of 250 words per minute, he could read 3,750 words
in fifteen minutes.250
1
=
x
15
,x=3, 750. Therefore he would be able to finish reading a
3,650-word article in fifteen minutes.
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