THE TRANSITION LINE // FIELD NOTES // STOP 27
Percent from data, complex area, and interest
Field Book
Lessons 101 to 104
Method
Pencil and paper. Calculators never.
The freight office runs the books: part over total, then the interest line. A part over a total turns into a percent through its decimal, complex areas fall to simple pieces, powers of ten with signed exponents slide the point both ways, and simple interest runs I = P · R · T on three lines.
What you have to be able to do
- Find a percent from a part and a total
- Find the area of a complex shape
- Multiply by powers of ten with positive and negative exponents
- Compute simple interest and a new balance
The procedures
Percent from part and totalField Book Lesson 101
- Write the part over the total as a fraction.
- Change it to a decimal, then to a percent.
Complex areaField Book Lesson 102
- Subdivide the shape into simple rectangles.
- Find each area, then add.
Powers of ten, both signsField Book Lesson 103
- A positive exponent moves the point right, one place per power.
- A negative exponent moves it left the same count.
Simple interestField Book Lesson 104
- I = P · R · T: principal times rate times time, on three lines.
- The new balance adds the interest to the principal.
Worked: the interest line
- I = P · R · T
- = 5000 · 0.04 · 3 = 600
- New balance: 5000 + 600 = 5600 dollars.
Worked: part over total
- 27 of the 60 cars are tankers: 2760 = 0.45.
- 0.45 = 45%
Terms on the record
| Term | Meaning |
|---|
| simple interest | interest that stays the same amount every year |
| principal | the amount of money being used or borrowed |