Ray's New Practical Arithmetic · Art. 183 (3d, 4th) · Unit 17: Interest
86. Interest for months and for days
Goal: Your child finds interest for a number of months or days by taking the right part of a year's or a month's interest, counting 30 days to the month.
You'll need
- Paper and pencil, or a slate or small whiteboard
What's in the book
Page 224 starts "When the Time is any Number of Months", with a worked example solved two ways and the rule, then problems. Page 225 does the same for days, and page 226 has the day problems. Answers are printed.
The lesson, step by step
Warm-up
Ask: What part of a year is 6 months? 3 months? 9 months? What part of a 30-day month is 15 days? 10 days?
Months
Find the interest of $24 for 9 mo., at 6%.
One year's interest is $1.44; 9 months is 3/4 of a year, and 3/4 of $1.44 is $1.08.
The book's second way uses parts: 6 mo. is half a year (72 ct.), 3 mo. is half of that (36 ct.), so the total is $1.08.
Days
Find the interest of $288 for 24 da., at 5%.
One month's interest is $1.20; 24 days is 4/5 of a month; 4/5 of $1.20 is 96 ct.
Read the custom that makes this work:
In computing interest, it is customary to regard 30 days as 1 month.
Guided practice
Do these two together:
Of $360 for 20 da., at 6%.
Of $1200 for 15 da., at 6%.
Independent practice and check
Your child works the months problems on pages 224–225 and the days problems on page 226. The book prints the answer at the end of each problem line. Have your child cover the answers with a strip of paper, work the problem, then slide the strip down to check.
Mental-math wrap-up
Aloud: interest on $100 at 6% for 6 months; for 2 months; for 15 days.
Tip
Old customs note: counting every month as 30 days (a 360-day year) was the usual business custom in Ray's day, and it makes the arithmetic much easier.