Think Ahead 2
Percentages in Agriculture
To prepare for the next week of class, you should fully understand and be able to answer the
following questions. You can check your answers with the answer key posted on your
instructor’s website or O drive to be sure you are correct. You can get help with this work from:
•
•
•
•
your instructor during office hours
the Algebra Alcove in Jones 147
PCS Center (the Math Tutoring Center) in EBH 271
online tutorials such as
http://www.wtamu.edu/academic/anns/mps/math/mathlab/beg_algebra/beg_alg_tut16_pe
rcent.htm
1. Write each term in the appropriate blank:
0.07, 7/100, cent, hundred, percent, per hundred
In the expression, 7%, the symbol % stands for ______________ . This means
“per______________”, since the word root ______________ means 100 (as in the word
centennial). So 7% means 7______________. This could be written as the fraction: ________ .
It could also be written using a decimal: ________ .
2. Here’s a simple percent problem: If you buy 12 books for the semester, and 3 of those books
were used, what percent of the books that you bought were used books?
You might be able to quickly answer “25%” (since 3 out of 12 is one fourth, or 25%), but it’s
important to remember why you can do this. Fill in the blanks in the following explanation.
!
!"
!
= !"" or “___ is to ___ as x is to 100” (because “per cent” means “___ __________”)
To solve this, 𝑥 = 100
!
!"
Or 𝑥 = so 𝑥 = %
3. A useful rule in working with percentages is, to change a number from a decimal to a percent
a) Move the decimal 1 place to the left
b) Move the decimal 2 places to the left
c) Move the decimal 1 place to the right
d) Move the decimal 2 places to the right
4. A useful rule in working with percentages is, to change a number from a percent to a decimal
a) Move the decimal 1 place to the left
b) Move the decimal 2 places to the left
c) Move the decimal 1 place to the right
d) Move the decimal 2 places to the right
Problem 5: percents in nutrition
A bowl of instant soup that I brought for my lunch says it contains 2 servings, but I of course ate
the whole thing.
Here is some of the label information:
Amount per serving
Calories 210
Calories from Fat 70
amount % of daily value
Total fat
8g
12
Saturated fat
3g
15
Trans fat
0g
Cholesterol
5mg
2
Sodium
830mg 35
Total carbohydrate 30g
10
Dietary fiber
1g
4
Protein
5g
a) First of all, since I ate the whole bowl, how many calories have I consumed? _____
b) And how many calories worth of fat have I consumed? _____
c) What percent of my total intake was from fat?
use
140
𝑥
=
420 100
d) Since I ate the whole bowl, how many grams of saturated fat have I eaten? _____
e) The label tells me that 3 g of saturated fat is 15% of the recommended daily intake of
saturated fat. So what is the recommended daily intake of saturated fat? _____
use
3𝑔
15
=
𝑥
100
f) What percentage of the calories I consumed came from the saturated fat?
The label actually doesn’t give you enough information to calculate this, does it?
g (grams) is a measure of the mass of fat I have eaten, but I have to be able to convert that mass
to calories. A good estimate is that 1 gram of fat has 9 calories in it.
Fill in the blanks: I ate ________ grams of saturated fat when I ate the whole bowl. So I ate
_______ calories worth of saturated fat. Now calculate the percent of my calories that came
from saturated fat.
g) If you know that 1 gram of protein is 4 calories, what percent of my calories (again, eating the
whole bowl) came from protein?
Problem 6: percents in the field
a) You just bought a 50 lb bag of ammonium nitrate fertilizer. It contains 34% nitrogen (N).
How many pounds of N are in the bag?
use
34
𝑥
=
100 50 𝑙𝑏𝑠
Though it’s the same calculation, some students will think of it this way:
0.34 50 𝑙𝑏𝑠 = 𝑥
b) How many pounds of the fertilizer would you need to obtain 5 pounds of nitrogen?
use
34
5𝑙𝑏𝑠
=
100
𝑥
c) If a 60 lb bag of ammonium nitrate fertilizer had 15 pounds of N, what is the percent of
nitrogen in the fertilizer?
use
𝑥
15
=
100 60