Answer:
13 mins 
Step-by-step explanation:
We need to find how many half lives are required to reach .0313 
.0313 = 1/2^n log both sides and solve for n 
log.0313 / log (1/2 ) = n where n= NUMBER of halflives 
n = ~ 5 half lives 
 these 5 half lives took up 65 minutes 
 so EACH half life is 65/5 = 13 mins