EAR and af

Dear sir,

Could you tell me whether my understanding about EAR and the n applied in the following situation is right?

The future cash inflow(4m) is paid half a year and the whole life period is 5 years.
The annual interest rate is 20%.

So the effective int. rate=(1+20%/2)2 -1=10.25%

Using the formula of af=[1 – (1 + r)(-n) ]/r

r=10.25%
n=10 (5*2)

then we get the af

Thanks in advance!!!