Can someone help me with this problem?

"at the end of each yr for the next 10 yrs you will receive $50. Initial investment is $320. what is the rate of return expected?"

I started by by looking at it this way

320 = 50 * (1-(1/(1+r)^10))/r

but impossible to solve...

p.s. i'm not allowed to use a financial calculator

do i just estimate it ..that will take forever and i'm not sure if there will be tables on the exam

Any suggestions?

Thanks.