Types of Simple Interest Questions
In this article, we are going to have a look at the various kinds of questions formed on Simple Interest.
Type 1: Find Simple Interest
Q. A man bought a mobile phone for Rs. 12,000. He gave Rs. 4,000 down payment and the rest after 20 months on which he was charged 10% per annum. What is the total amount he paid for the mobile phone?
(a) Rs. 12,000
(b) Rs. 13,000
(c) Rs. 13,333.33
(d) Rs. 14,000
Explanation:
Cost of the mobile phone = Rs. 12,000
Down payment = Rs. 4,000
Hence, the money still to be paid = 12000 – 4000 = Rs. 8,000
Time after which the rest of the amount is paid = 20/12 years = 5/3 years
Simple Interest = (Principle × Rate of interest × Time duration)/100 = [8000× 10 × (5/3)]/100 = Rs. 1,333.33
Hence, the total money still due to be paid = 8000 + 1333.33 = Rs.9,333.33
So, the total cost of mobile paid = 9333.33 + 4000 = Rs. 13,333.33
Answer: (c)
Type 2: Find r when Amounts given
Type 2a
Q. Rohan invested a sum of money in a simple interest bond. The money grew to Rs. 4000 in 3 years and to Rs. 5600 in 7 years. What was the rate of interest?
(a) 12.36% (b) 14.28% (c) 15% (d) 17.68%
Explanation:
Let the principal amount be Rs. X
Interest earned in 3 years = Amount at the end of 3 years – principal amount = 4000 – X
Interest earned = principal amount × interest rate × time in years
or 4000 – X = X × interest rate × 3
or Interest rate = (4000 – X) /(3X)
Interest earned in 7 years = Amount at the end of 7 years – principal amount = 5600 – X
Interest earned = principal amount × interest rate × time in years
or 5600 – X = X × interest rate × 7
or Interest rate = (5600 – X) /(7X)
But interest rate is the same in both the cases. So we can equate two equations to get the value of X
i.e. (4000 – X) / (3X) = (5600 – X) / (7X)
or 28000 – 7X = 16800 – 3X
or X = Rs. 2,800
Interest rate (%) = [(5600 – X) /(7X)] × 100 = 100 / 7 = 14.28 %
Answer: (b)
Type 2b
Q. Nicole invests a certain amount of money for two years at simple interest. He gets a total of Rs. 2,400 at the end of two years when annual rate of interest is 10%. What should be the rate of interest to get a total sum of Rs. 3,200 at the end of two years?
(a) 20% (b) 30% (c) 24% (d) 32%
Explanation:
Let principle be Rs. P
Simple Interest = Total Amount – Principle = 2400 – P
Now, Simple Interest = (P × r × t)/100
Or 2400 - P = (P × 10 × 2)/100
Or P = Rs. 2,000
If the total amount = Rs. 3,200, then Simple interest = 3200 – 2000 = Rs. 1,200
Now, Simple Interest = (P × r × t)/100
Or 1200 = (2000 × r × 2)/100
Or r = 30%
Answer: (b)