Question

A certain sum doubles in 7 years at simple interest. The same sum under the same interest rate will become 4 times in how many years.
(a) 14
(b) 28
(c) 21
(d) 10

Answer 1

Hey there!!

Let us take the principle account as ' x '

Interest = 2x

Term = 7 years

Formula = ( principle ) × ( rate ) × ( term ) ÷ 100

2 x = ( x ) × ( rate ) × ( 7 ) ÷ 100 ( let us take rate as y )

200x = ( 7 ) ( y ) ( x )

200x ÷ x = 7y

200 = 7y

y = 200 ÷ 7

Rate = 200 ÷ 7

The interest now has to become 4x and we will have to find the term

Take term as y

4x = ( x ) × ( 200 ) × ( y ) ÷ ( 100 × 7 )

4x = 2xy ÷ 7

28x = 2xy

28 = 2y

y = 14 years

Term = 14 years

_________________

Answer 2

Answer:

21 Years

Step-by-step explanation:

let principal be Rs.100

As per data given, sum becomes doubles means,

Amount is Rs.200 , it simply means interest add to principal is Rs.100.

In simple interest process, every time period we get same interest so, the same principal to become 4 times means to become Rs.400, it should added with Rs.300.

For every 100 it takes 7 years, so for 300 it takes 21 Years.

or

P(100)------A(200)  so interest is 100 and takes 7 years

P(100) -----A(400) here interest is 300.

It takes 7 years for 100(interest)

now for how many years it takes 300(interest)

Answer is: 21 years