Ray's New Primary Arithmetic · Lesson LXXVII · Unit 7: Signs, bigger numbers and mixed problems
77. Trading, work and "if this, then that"
Goal: Your child solves problems where more workers take less time, and "if 7 cost this, what do 5 cost".
You'll need
- A slate, small whiteboard or paper and pencil
What's in the book
Problems such as "If 4 men can mow a field in 5 days, in how many days can 10 men mow it?" and "If 7 barrels of flour cost 84 dollars, what will 5 barrels cost?"
The lesson, step by step
Work problems
4 men take 5 days, so the field is 20 "man-days" of work. 10 men do it in 20 ÷ 10 = 2 days. Act it out with toy figures.
Go through one
For the flour: 1 barrel costs 84 ÷ 7 = 12 dollars, so 5 cost 60 dollars.
Work through the page
Do the problems, over two days if needed.
Afterwards
These are the hardest problems in the book. It is fine to come back to them later.
Answers to the book's problems
7. 2 days. 12. 12 days. 13. 60 dollars. 14. 12 trees in each row. (Work the others together; the scan is hard to read in places.)
Worked out for this site from the scan. If a number in your copy differs, trust the book.