Seed X costs $5 per pound and Seed Y costs $7 per pound. Seed Z is a

This is a question of Weighted averages where X costs 5 $/pound and Y costs 7 $/pound

When a particular amount of X is added to a particular amount of Y, mixture Z results

The more the proportion of X or Y determines the inclination of average price of mixture towards per pound $ price of X or Y

S1: it says that Mixture Z costs less than 6 $ /pound (because 20 pounds of Z costs less than 120$ => per pound cost less than 6$) implying that the mixture Z should have more of X (which costs $5 /pound) than Y (which costs $7/pound) in order to make the weighted average price tilt towards the $/pound price of X (being 5$/pound) .

Hence a (proportion of X) has to have a higher number as compared to b (proportion of Y) in order to have the mixture Z cost less than $ 6/pound . Now weight of mixture Z is 20 pounds. Hence a>10 . Sufficient


S2: b<12 . This statement only talks about proportion of X and Y in Z (without mentioning anything on price)
if b=3, a=17 , a>10- yes
if b=11, a=9 , a/\>10- no

Insufficient

Hence Answer is A