Theory & Concepts
SSC CGL Simple Interest Questions, Formulas & Short Tricks
Prepare Simple Interest for SSC CGL with formulas, short tricks, solved examples, practice questions, PYQs, and free PDF notes for faster exam preparation.
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Practice Notes
20 min readDifficulty: Beginner to Intermediate
Simple interest (si) is a fundamental topic in the Ssc cgl quantitative aptitude section. It forms the basis for more complex topics like compound interest and installments. Unlike compound interest, simple interest is calculated only on the principal amount for the entire duration.
1. Essential formulas
Simple interest (si)
Total amount (a)
2. Solved examples
Question 01Standard pattern
A sum of money amounts to rs. 520 in 5 years and to rs. 568 in 7 years at simple interest. Find the principal.
rs. 400
rs. 410
rs. 380
rs. 420
Correct answer: a) rs. 400
Solution
Interest for 2 years (7-5) = \( 568 - 520 = rs. 48 \).
Interest for 1 year = \( \frac{48}{2} = rs. 24 \).
Interest for 5 years = \( 24 \times 5 = rs. 120 \).
Principal = \( 520 - 120 = rs. 400 \).
Question 02Standard pattern
At what rate of simple interest per annum will a sum of money double itself in 10 years?
10%
12%
8%
15%
Correct answer: a) 10%
Solution
Rule: \( R = \frac{(n - 1) \times 100}{T} \).
Here \( n = 2 \) and \( T = 10 \).
Calculation: \( R = \frac{(2 - 1) \times 100}{10} \).
Final rate = \( 10\% \).
Question 03Standard pattern
A sum of rs. 12,500 amounts to rs. 15,500 in 4 years at a certain rate of simple interest. If the rate of interest is increased by 3%, what will be the new amount?
rs. 17,000
rs. 16,500
rs. 17,200
rs. 16,800
Correct answer: a) rs. 17,000
Solution
Existing amount = \( rs. 15,500 \).
Additional interest = \( \frac{12500 \times 3 \times 4}{100} \).
Additional interest = \( 125 \times 12 = rs. 1,500 \).
New amount = \( 15500 + 1500 = rs. 17,000 \).
Question 04Standard pattern
In how many years will a sum of rs. 3,000 yield a simple interest of rs. 1,080 at 12% per annum?
3 years
2 years
4 years
2.5 years
Correct answer: a) 3 years
Solution
Use formula: \( T = \frac{SI \times 100}{P \times R} \).
Calculation: \( T = \frac{1080 \times 100}{3000 \times 12} \).
Simplify: \( \frac{108000}{36000} = 3 \).
Result = \( 3 \text{ years} \).
Question 05Standard pattern
A person lends 40% of his sum of money at 15% per annum, 50% of the rest at 10% per annum and the rest at 18% per annum. What would be the annual rate of interest?
14.4%
13.2%
15%
12.8%
Correct answer: a) 14.4%
Solution
Let total sum be \( 100 \) units.
Interest 1: \( 40 \text{ units at } 15\% = 6 \text{ units} \).
Interest 2: \( 50\% \text{ of } 60 = 30 \text{ units at } 10\% = 3 \text{ units} \).
Interest 3: \( 30 \text{ units at } 18\% = 5.4 \text{ units} \).
Total interest = \( 6 + 3 + 5.4 = 14.4\% \).
Question 06Standard pattern
The simple interest on a certain sum for 8 months at 4% per annum is rs. 129 less than the simple interest on the same sum for 15 months at 5% per annum. Find the sum.
rs. 3,600
rs. 4,200
rs. 3,000
rs. 4,000
Correct answer: a) rs. 3,600
Solution
Time units: \( T_1 = \frac{2}{3}, T_2 = \frac{5}{4} \).
Interest difference: \( \left( 5 \times \frac{5}{4} \right) - \left( 4 \times \frac{2}{3} \right) = \frac{43}{12} \text{ units} \).
Equation: \( \frac{\text{Sum} \times 43}{1200} = 129 \).
Sum = \( \frac{129 \times 1200}{43} = 3 \times 1200 = rs. 3,600 \).
Question 07Standard pattern
A sum was put at simple interest at a certain rate for 2 years. Had it been put at 3% higher rate, it would have fetched rs. 72 more. Find the sum.
rs. 1,200
rs. 1,500
rs. 1,000
rs. 2,000
Correct answer: a) rs. 1,200
Solution
Additional rate effect = \( 3\% \times 2 = 6\% \).
Value: \( 6\% \text{ of Sum} = rs. 72 \).
1%: \( \frac{72}{6} = rs. 12 \).
Sum: \( 12 \times 100 = rs. 1,200 \).
Question 08Standard pattern
What annual payment will discharge a debt of rs. 6,450 due in 4 years at 5% simple interest?
rs. 1,500
rs. 1,600
rs. 1,450
rs. 1,550
Correct answer: a) rs. 1,500
Solution
Debt = \( nP + \frac{Prn(n-1)}{200} \).
Substitute: \( 6450 = 4P + \frac{P \times 5 \times 4 \times 3}{200} \).
Simplify: \( 6450 = 4.3P \).
Result: \( P = rs. 1,500 \).
Question 09Standard pattern
If rs. 64 becomes rs. 83.20 in 2 years, what will rs. 86 become in 3 years at the same rate of interest?
rs. 124.70
rs. 115.50
rs. 118.20
rs. 120.40
Correct answer: a) rs. 124.70
Solution
Interest case 1 = \( 83.20 - 64 = rs. 19.20 \).
Rate = \( \frac{19.20 \times 100}{64 \times 2} = 15\% \).
Interest case 2 = \( \frac{86 \times 15 \times 3}{100} = rs. 38.70 \).
New amount = \( 86 + 38.70 = rs. 124.70 \).
Question 010Standard pattern
A sum of rs. 10 is lent to be returned in 11 monthly installments of rs. 1 each. What is the rate of interest?
21(9/11)%
10%
12%
20%
Correct answer: a) 21(9/11)%
Solution
Interest paid = \( rs. 1 \).
Sum of outstanding principal = \( 55 \).
Formula: \( 1 = \frac{55 \times R}{1200} \).
Rate = \( \frac{1200}{55} = 21\frac{9}{11}\% \).
