Year 2 Lesson Plan 20 - Working
- (5 min) Mental Math
- 38% of $1 is how much? [38 cents]
- 100% of $1 is how much? [All of it = $1]
- 138% of $1 is how much? [$1.38]
- 138% of $10 is how much? [$13.80]
- Find 30% of 40, then add 25 
- (5 min) Review of selected problems from previous lesson (no more
than 3 problems)
- Hand out the Fractions, Decimals, and Percents reference sheet.
Review how to convert decimals to percents and back by moving the decimal
- Setting up and solving Percent problems
When you have to solve a problem involving percents:
- Write down this equation: D x N = A
D is the percent, changed into a decimal
N is the amount you are working on
A is the answer. It can be larger than N if D is larger than 1 (100%)
- If you are given a percent, change it to a decimal (D)
- Figure out whether you are missing D, N or A
- Put the other two that you have into the equation
- Solve for the missing number
- Convert D back to a percent if that was what you are asked for
- REMEMBER: of means multiply and is means "="
- EXAMPLE 1--Find the answer: What is 8% of 752?
Well, D = .08 and N = 752 and A is the missing number, so
.08 x 752 = A
A = 60.16
- EXAMPLE 2--Find the percent: 64 is what percent of 512?
Here you are given the answer and must find the percent:
D is the missing number, N = 512 and A = 64
D x 512 = 64
D = 64/512 = .125 or 12.5%
- EXAMPLE 3--Find the "mystery" number: 125% of what number is
D = 1.25, N is the missing number, and A = 15
1.25 x N = 15
N = 15/1.25 = 12
Does this seem reasonable? Yes! We are told 15 is more than 100%
of the number, so the number must be smaller than 15.
- Extending the concept
- Discount means you subtract a percent from the original
- If a $100 bicycle is discounted 25%, how much do you pay? [$75]
- If Lamont's has a 30% off sale, how much do you pay for $30
shoes? [$21] Not 20, that would be 1/3 off.
- Tax or interest means you add a precent to the original amount.
You pay tax after you take any discounts.
- If you buy a $100 bicycle with 8.2% tax, how much do you pay?
- If you borrow $100 at 5% interest, how much do you have to pay back?
[$105] This is called simple interest.
- Interest is charged over time. If you borrow $100 for 2
years at 5% interest per year, how much do you have to pay back? Well, the
first year you owe $105. The second year you owe $105 + 5% of 105, or
$5.25. So after 2 years you owe$110.25. This is called compound
interest. This mounts up after a long time with big loans. If your
parents borrowed $100,000 for a house at 5% interest for 30 years, they
would have to pay back $193,255 in all!
- (Remainder of class) In-class exercise
- Hand out homework as students successfully complete the in-class exercise.