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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  ୖ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
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