: the the sample is represented by
400 tickets, 131 marked 1 and the rest marked 0.  We estimate the
SD of the box by that of the sample. So

 	
 SD = ( 121/400 x 279/400 )^(1/2) = .46 

The SE for the number of 1's in a sample of size 400 is
approximately 
 	
 .46 x 20 = 9.2 

so the SE for the percentage of 1's in such a sample is  
 	
 (9.2/400) x 100% = 2.3%. 

The EV for the number of 1's is 
 	
 (121/400) x 100% = 30%. 

The z-value corresponding to 70% is approximately 1.05, and

 	
 1.05 x 2.3% = 2.4%. 

The 70% confidence interval runs from approximately 
 	
 30 - 2.4 = 27.6% 

to

 	
 30 + 2.4 = 32.4%. 

Of the answers, "between 28% and 32%" is the closest.