# Math Help - Compounding quarterly!

1. ## Compounding quarterly!

find the time required for an investment of 5000 dollars to grow to 9000 dollars at an interest rate of 7.5 percent per year, compounded quarterly.

t= _____

this is what i have got so far please correct me if i am wrong

9000=5000(1+.075/4)^4t

so you divide by 5000 on each side leaving you with
9000/5000= 1.8

1.8= (1+.075/4)^4t

Ln1.8 = Ln(1+.075/4)^4t

divide by 4(1+.075/4)

ln1.8/4ln(1+.075/4) = t

then whats next? cuz i got that this above part equals 7.9

so then i replugged it back into the first equation but it says its not correct

8993.04676

used 7.9 ??

2. Originally Posted by evinez
find the time required for an investment of 5000 dollars to grow to 9000 dollars at an interest rate of 7.5 percent per year, compounded quarterly.

t= _____

this is what i have got so far please correct me if i am wrong

9000=5000(1+.075/4)^4t

so you divide by 5000 on each side leaving you with
9000/5000= 1.8

1.8= (1+.075/4)^4t

Ln1.8 = Ln(1+.075/4)^4t

divide by 4(1+.075/4)

$\ln1.8/{\color{red}(} 4\ln(1+.075/4)/{\color{red})} = t$

then whats next? cuz i got that this above part equals -.584
If you use brackets then you'll get $t =7.9$