Ray's New Intellectual Arithmetic · Lesson LXXIII · Unit 8: Percentage and interest
73. Interest for months and days
Goal: Your child finds interest for parts of a year.
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
Twenty-one problems, with models: 6 months is ½ a year, so 6 per cent a year is 3 per cent; and $120 for 6 months 15 days at 6 per cent is $3.90.
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
SayAt 6 per cent a year, 1 month is ½ per cent. For 6 months that is 3 per cent. 3 per cent of $50 is $1.50.
The old shortcut: at 6 per cent, every 2 months is 1 per cent.
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:
What is the interest on $50 for 6 months at 6 per cent?
Find the interest on $120 for 6 months 15 days at 6%.
Find the interest on $200 for 4 months 24 days at 6%.
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
1) $1.50. 2) $1. 3) $2.80. 4) $2.40. 5) $4.50. 6) $3.90. 8) $3.53. 9) $4.80.
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 ↗.