Simple & Compound Interest

[sasidhardiamond]

Q) A certain sum doubles in 7 years.

Column-A

the time taken for the principal to become 4 times when compounded annually

 

Column-B

the time taken for the principal to become 3 times under simple interest.

 

 

-Please explain in detail-

[kanika]

Is the answer C

[shikamaru]

Is the answer C

 

Plz explain the procedure

[sasidhardiamond]

answer is either A or B

[shikamaru]

Ans is B

 

Col A : 10 to 11 (10.38 exact, i couldn't do that with out a help of a calculator, plz suggest a easy way)

Col B :14

 

From given condition

2P=P(1+r*7) => r = 1/7

 

for col b : same equation , we get n = 14

for col a : 4P=P (1+r)^n , n => 10.38 (this is part i needed calculator for :( but i could guess it was between 10 and 11)

 

Correct me if i am wrong, btw we are considering in the given condition, the interest rate is simple ..is this valid?

[kanika]

shikamaru for column A they have asked the Compuond Interest so it comes out to be 14

For B they have asked SI whose formulae is p*r*t/100

[shikamaru]

shikamaru for column A they have asked the Compuond Interest so it comes out to be 14

For B they have asked SI whose formulae is p*r*t/100

 

i think thats what i have done. or am i missing something? are you referring that given condition is compound interest?

[kanika]

for col b : same equation , we get n = 14( which is answer for col A)

for col a : 4P=P (1+r)^n , n => 10.38 (this is part i needed calculator for :( but i could guess it was between 10 and 11) (for Col B be have to apply calculate the rate then apply the SI

[shikamaru]

"Column-A

the time taken for the principal to become 4 times when compounded annually"

 

this is compound interest. equation => A=P(1+r/k)^(kt) => here k=1;t=? and we should get the value of "r=interest rate" from given condition.

 

Given condition >

A certain sum doubles in 7 years.

 

it didn't mention if it was in simple or compound interest. my confusion lies here.

 

i think its better you describe your solve, that way i can understand whats wrong with mine.

[sasidhardiamond]

I understood that, the sum becomes 3 times under simple interest in 14 years

but, how to calculate for the compound interest-for the sum to double three times...