Page 124 - ICSE Math 7

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Example 7: Find the principal that will amount to ` 4,826 in 6 years at the rate of 4 % per annum.
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Solution: Let the principal be ` 100.

P × R × T
S.I. = 100

100 × 9 × 6
\ S.I. = 100 × 2 ⇒ S.I. = ` 27

A = P + S.I. ⇒ A = ` 100 + ` 27 = ` 127
When amount is ` 127, principal is ` 100.

 100
\ When amount is ` 1, principal is `  127  .


When amount is ` 4,826, principal is `  100 × ,  = ` 3,800.
4 826



127
Alternatively,
Let the principal be ` x. Try These
P × R × T
S.I. = 100 1. Simran has ` 28,000 in her account and
receives 5% rate of interest. After how many
x × 9 × 6 27x years would she earn ` 420 as interest on this
amount?
\ S.I. = 100 × 2 ⇒ I = 100 2. What sum should be borrowed at the rate of
27x 5% p.a. for 4 years to produce the same interest
A = P + S.I. ⇒ 4,826 = x + as ` 1,500 borrowed at 3% for 4 years?
100
127x
⇒ 4,826 = 100 ⇒ x = 3,800

Thus, the principal is ` 3,800.
Example 8: A sum of money invested at 6% p.a. amounts to ` 826 in 3 years. How much would it
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amount to in 6 years at 10 % p.a.?
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Solution: Let the sum of money invested (principal) be ` 100.

P × R × T
S.I. = 100

100 × 6 × 3
\ S.I. = 100 = ` 18

A = P + S.I. = ` 100 + ` 18 = ` 118
When amount is ` 118, principal is ` 100.

 100 
\ When amount is ` 1, principal is `   .
 118 

 100 826× 
When amount is ` 826, principal is `   = ` 700.
 118 
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Now, P = ` 700, R = 10 % p.a. = 2 % p.a., T = 6 years
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