Page 24 - Mathematics-for-the-Senior-Phase-Grades-7-9_Neat
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Senior Phase Mathematics: Grade 7 – 9 Learner book
Unit 1.7: Simple and compound interest
Simple interest
Simple interest is interest charged (or earned) on the initial amount borrowed (or invested).
Worked example:
If I invest R500 in an account that pays 10% simple interest per year, I will earn 10% of Grade 7, 8 and 9
R500 = ଵ଴ ൈ R500 ൌ R50 as interest each year. So if I invest it for 3 years, I’ll earn
ଵ଴଴
3 ൈ R50 ൌ R150 in interest. So at the end of 3 years, I’ll have R650 in total.
We use the formula:
A ൌ Pሺ1 ൅ ݅݊ሻ A = total paid back (loan) or accumulated (investment)
SI ൌ P. ݊. ݅ SI = simple interest charged (loan) or earned (investment)
P = amount borrowed (loan) or invested (investment)
Ý… = interest rate
ÝŠ = number of years
Hire Purchase
Customer wants → Store allows → Store charges Customer pays
to buy something customer to pay simple interest on off the loan plus
loan and calculates → interest in
expensive but a deposit and monthly amounts
doesn’t have cash remaining how much the
customer owes Grade 8 and 9
to pay for it amount is a loan
immediately
Worked example:
I buy a fridge on hire purchase for R3 000. I pay a 10% deposit. The store
charges 25% interest per year and allows me to pay it back over 2 years.
How much do I need to repay each month?
Deposit = 10% of R3 000 = R300
So hire purchase loan = R3 000 – R300 = R2 700
Amount I need to repay including interest:
A ൌ Pሺ1 ൅ ݅݊ሻ P ൌ R2 700; ݅ ൌ 25% ൌ 0,25; ݊ ൌ 2
A ൌ R2 700ሺ1 ൅ 0,25 ൈ 2ሻ ൌ R4 050
R4 050 over 24 months is à–ସ଴ହ଴ ൌ R168,75 per month.
ଶସ
Compound interest
Compound interest is interest charged (or earned) on the initial amount, as well as on any
interest already charged (or earned).
We use the formula: A = total paid back (loan) or accumulated (investment) Grade 9
P = amount borrowed (loan) or invested (investment)
A ൌ Pሺ1 ൅ ݅ሻ௡ ݅ = interest rate
ÝŠ = number of years
Worked example:
I invest R5 000 in the bank for 6 years and they pay me 8% compound interest
per year. How much will I have in the bank after 6 years?
A ൌ Pሺ1 ൅ ݅ሻ௡ P ൌ R5 000; ݅ ൌ 8% ൌ 0,08; ݊ ൌ 6
A ൌ R5 000ሺ1 ൅ 0,08ሻ଺
ൌ R5 000ሺ1,08ሻ଺ ൌ R7 934,37
Winning Teams & OLICO 21

