Finding a Number When the Percent Is Known

1
Finding a Number When the
Percent Is Known
8-3
Warm Up
Problem of the Day
Lesson Presentation
Pre-Algebra
2
Warm Up 1. What percent of 20 is 18? 2. What
percent of 400 is 50? 3. 9 is what percent of
27? 4. 25 is what percent of 4?
90
12.5
625
3
60 is 12 of what number?
Set up an equation to find the number.
60 12 ? n
60 0.12n
500 n
60 is 12 of 500.
4
75 is 25 of what number?
Set up an equation to find the number.
75 25 ? n
75 0.25n
300 n
75 is 25 of 300.
5
A. A fisherman caught a lobster that weighed 11.5
lb. This was 70 of the weight of the largest
lobster that fisherman had ever caught. What was
the weight, to the nearest tenth of a pound, of
the largest lobster the fisherman had ever caught?
Think 11.5 is 70 of what number?
11.5 70 ? n
The largest lobster the fisherman had ever caught
was about 16.4 lb.
11.5 0.70 n
16.4 ? n
6
B. When a giraffe is born, is approximately 55
as tall as it will be as an adult. If a baby
giraffe is 5.2 feet tall when it is born, how
tall will it be when it is full grown, to the
nearest tenth of a foot?
Think 55 of what adult height is 5.2?
5.2 55 ? h
This giraffe will be approximately 9.5 feet tall
when full grown.
5.2 0.55 h
9.5 ? n
7
A. Amy caught a fish that weighed 15.5 lb. This
was 85 of the weight of the largest fish that
Amy had ever caught. What was the weight, to the
nearest tenth of a pound, of the largest fish
that Amy had ever caught?
Think 15.5 is 85 of what number ?
15.5 85 ? n
The largest fish that Amy had ever caught was
about 18.2 lb.
15.5 0.85 ? n
18.2 ? n
8
You have now seen all three types of percent
problems.
15 of 120 n
p of 120 18
15 of n 18
9
Lesson Quiz
80
1. 10 is 12 of what number? 2. 326 is 25
of what number? 3. 44 of what number is 11? 4.
290 of what number is 145? 5. Larry has 9 novels
about the American Revolutionary War. This
represents 15 of his total book collection. How
many books does Larry have in all?
1304
25
50
60
10
Homework

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