Question 1112350
9000=4000(1+.07/12)^12n, from formula P=Po(1+r/n)^nt
(9/4)=(1.005833)^12n
ln(9/4)=12n log (1.005833), without rounding that initially
12n=139.42, and without rounding, divide by 12 to get 11.62 years, rounding here.
Rule of 72 would say it would double in 10.3 years, so this makes sense.

P=Po(1+(.12/4)^4n, P/Po=2
2=(1.03)^4n
ln2=4n ln (1.03)
4n=23.44, n=5.86 years; rule of 72 would say 6 years.