Find the doubling time of a quantity that is increasing by 9.8% per year.

Question

haseeb96 · Answer

lets suppose 
time = x 
then double time = 2x okay ???

anonymous · Answer

t = log 2/log 1.08 = 9.006468 years

anonymous · Answer

Why log 1.08?

welshfella · Answer

yea  where did you get that from?

anonymous · Answer

Somebody help, I'm confused :(

anonymous · Answer

i gave you the answer i cant give you the show work to

welshfella · Answer

oops guys I must apologise
I had it the wrong way round!

you divide the rate into 72
so it is

72 / 9.8 = 7.35 years

anonymous · Answer

That's wrong @welshfella

anonymous · Answer

@kinggg  How did you get log 1.08? Where did the number 1.08 come from?

welshfella · Answer

how do you know its wrong?

anonymous · Answer

I entered it and it said answer is NOT CORRECT

anonymous · Answer

@kinggg 9.006468 years is incorrect too

welshfella · Answer

i think the log formula is
ln2 /  ln(1.098)

welshfella · Answer

that gives 7.41 years

anonymous · Answer

7.41 years is correct!

welshfella · Answer

Lets do it the long way teh
suppose the principle is $100
we have 

double amount 200 = 100(1 + 0.098)^n

(1.098)^n = 2
take logs:-

n ln 1.098 = ln
n = ln 2 / ln1.098) = 7.41

anonymous · Answer

are you doing Exponential growth, doubling time

welshfella · Answer

rounding that up its 8 years

anwaarullah · Answer

8 years    ,, let   100 be the investment so 100*1.098=,,,,,

welshfella · Answer

yea

welshfella · Answer

i guess that the interest is paid at the end of the year  so that would make it 8.

anonymous · Answer

Thanks all!

welshfella · Answer

yw