To prove that 2/3root5 is irrational

i=prt 
where i is interest (9000-6000 = $3000 interest) 
p is principle ($6000) 
r is rate (5.5%) [taking that its 5.5%pa] 
t is time 

substitute these into the equation 
3000 = $6000 x 5.5% x t 
t = 3000 divided by (6000x5.5%) 
= 3000 divided by 330 
t=100/11 years 
 it would take 100/11 years which is approx 9.1 years