Question 185731
I'm assuming that you want to solve for "r" right? If so, then I'll start where you left off.



{{{5000=250(1+r/4)^40}}} Start with the given equation



{{{5000/250=(1+r/4)^40}}} Divide both sides by 250



{{{20=(1+r/4)^40}}} Reduce



{{{root(40,20)=1+r/4}}} Take the 40th root of both sides (this is probably where you got stuck)



{{{1.0778=1+r/4}}} Evaluate the 40th root of 20 to get roughly 1.0778



{{{1.0778-1=r/4}}} Subtract 1 from both sides.



{{{0.0778=r/4}}} Subtract



{{{0.0778*4=r}}} Multiply both sides by 4 to isolate "r"



{{{0.3112=r}}} Multiply



So the answer is {{{r=0.3112}}} which means that the interest rate is 31.12% (just multiply by 100 to get a percentage)