model 1 basic compound interest using formula Section-Wise Topic Notes With Detailed Explanation And Example Questions
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The following question based on compound interest topic of quantitative aptitude
(a) 20%
(b) 5%
(c) 15%
(d) 10%
The correct answers to the above question in:
Answer: (d)
Using Rule 1,
Let the sum be P and rate of interest be R% per annum. Then,
$P(1 + R/100)^2 = 9680$ ...(i)
$P(1 + R/100)^3 = 10648$ ...(ii)
On dividing equation (ii) by (i)
$1 + R/100 = 10648/9680$
$R/100 = 10648/9680$ -1
= ${10648 - 9680}/9680$
$R/100 = 968/9680 = 1/10$
R = $1/10 × 100$ = 10%
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Read more basic problems using formula Based Quantitative Aptitude Questions and Answers
Question : 1
In what time will Rs.1000 becomes Rs.1331 at 10% per annum compounded annually ?
a) 3$1/2$ years
b) 3 years
c) 2 years
d) 2$1/2$ years
Answer »Answer: (b)
Using Rule 1,
Let the required time be n years.
Then,
1331 = 1000$(1 + 10/100)^n$
[$P_1 = P(1+ r/100)^n$]
$1331/1000 = ({10 + 1}/10)^n$
$(11/10)^n = (11/10)^3$
n = 3
Question : 2
The compound interest on Rs.16,000 for 9 months at 20% per annum, interest being compounded quarterly, is
a) Rs.2,518
b) Rs.2,520
c) Rs.2,522
d) Rs.2,524
Answer »Answer: (c)
Using Rule 1,
The interest is compounded quarterly.
R = $20/4$ = 5%
Time = 3 quarters
C.I. = P$[(1 + R/100)^T - 1]$
= 16000$[(1 + 5/100)^3 - 1]$
= $16000[(21/20)^3 - 1]$
= $16000({9261 - 8000}/8000)$
= $16000 × 1261/8000$ = Rs.2522
Question : 3
A sum of Rs.8000 will amount to Rs.8820 in 2 years if the interest is calculated every year. The rate of compound interest is
a) 5%
b) 6%
c) 3%
d) 7%
Answer »Answer: (a)
Using Rule 1,
If the rate of C.I. be r% per annum, then
A = P$(1 + R/100)^T$
8820 = 8000$(1 + r/100)^2$
$8820/8000 = (1 + r/100)^2$
$441/400 = (21/20)^2 = (1 + r/100)^2$
$1 + r/100 = 21/20$
$r/100 = 21/20 - 1 = 1/20$
r = $1/20$ × 100
r = 5% per annum
Question : 4
At what rate per annum will Rs.32000 yield a compound interest of Rs.5044 in 9 months interest being compounded quarterly ?
a) 80%
b) 20%
c) 50%
d) 32%
Answer »Answer: (b)
Using Rule 1,
Let the rate of CI be R per cent per annum.
CI = P$[(1 + R/100)^T - 1]$
5044 = 32000$[(1 + R/400)^3 - 1]$
[Since, Interest is compounded quarterly]
$5044/32000 = (1 + R/400)^3 - 1$
$(1 + R/400)^3 - 1 = 1261/8000$
$(1 + R/400)^3 = 1 + 1261/8000$
$(1 + R/400)^3 = 9261/8000 = (21/20)^3$
1 + $R/400 = 21/20$
$R/400 = 21/20 - 1 = 1/20$
R = $400/20$ = 20
Question : 5
A sum becomes Rs.1,352 in 2 years at 4% per annum compound interest. The sum is
a) Rs.1,250
b) Rs.1,225
c) Rs.1,245
d) Rs.1,270
Answer »Answer: (a)
Using Rule 1,
Let the sum be Rs.x.
1352 = $x(1 + 4/100)^2$
1352 = $x(1 + 1/25)^2$
$1352 = x(26/25)^2$
$x = {1352 × 25 × 25}/{26 × 26}$ = Rs.1250
Question : 6
The compound interest on Rs.6,000 at 10% per annum for 1$1/2$ years, when the interest being compounded annually, is
a) Rs.900
b) Rs.910
c) Rs.930
d) Rs.870
Answer »Answer: (c)
Amount = 6000$(1 + 10/100) × (1 + {{1/2} × 10}/100)$
= $6000 × 11/10 × 21/20$ = Rs.6930
Using Rule 4,If the time is in fractional form i.e.,t = nF, thenA = P$(1 + r/100)^n(1 + {rF}/100)$e.g. t =3$5/7$ yrs, thenA = P$(1 + r/100)^3(1 + r/100 × 5/7)$
Here, t = nF
A = P$(1 + r/100)^n(1 + {rF}/100)$
CI = Rs.(6930 - 6000) = Rs.930
compound interest Shortcuts and Techniques with Examples
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model 1 basic compound interest using formula
Defination & Shortcuts … -
model 2 at ci sum becomes ‘n’ times after ‘t’ years
Defination & Shortcuts … -
model 3 combination of si & ci
Defination & Shortcuts … -
model 4 difference in ci & si
Defination & Shortcuts … -
model 5 ci with instalments
Defination & Shortcuts … -
model 6 comparing sum in different years
Defination & Shortcuts …
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