Ray's New Intellectual Arithmetic · Lesson LXXII · Unit 8: Percentage and interest
72. Simple interest for whole years
Goal: Your child finds the interest on money for whole years.
You'll need
- The book page (press “📖 Book page”, or use a printed copy)
- Paper for jotting, though the work is meant to be done in the head
What's in the book
Definitions of interest, principal and amount, a model (the interest on $2 for 3 years at 5 per cent is 30 cents), and ten problems.
The lesson, step by step
Warm up
Ask three or four problems from the last lesson again, out of order. Your child answers and says why.
Show the reasoning
Say5 per cent a year for 3 years is 15 per cent. 15 per cent of $2 is 30 cents.
The amount is the principal and the interest together: $2.30.
Work the problems aloud
Read each problem on the page aloud. Your child answers, then explains in a full sentence, the way the book's model solutions do. The problems go like this:
Find the interest on $40 for 4 years at 5%.
Find the interest on $60 for 2 years at 7%.
Find the interest on $80 for 5 years at 9%.
Ask “Why?”
After each answer ask “Why?” or “How do you know?” Ray's wants the reasoning said aloud, not just the number. If your child stalls, go back to the model and say the first half of the explanation together.
Wrap up
Pick the two problems that were hardest today. Tomorrow, start with those as the warm-up.
Afterwards
Do the rest of the lesson's problems over the next day or two, a few at a time. Move on when your child can explain them without help.
Answers to the book's problems
6) $8. 7) $9. 8) $8.40. 9) $9. 10) $36.
Worked out for this site from the scan. If a number in your copy differs, trust the book. Every answer is in Ray's own Key ↗.