Sarah bought bananas, apples and pears. the ratio of bananas to apples was 3 to 5 and the ratio of apples to pears was 4 to 2. she bought 12 bananas. how many pears did she buy?

If Sarah buys 3 bananas for every 5 apples and bought 12 bananas that means she bought 20 apples. (3/5 = 12/A  solve and A=20)   
Now we know that for every 4 apples she buys 2 pears.  Because she bought 20 apples, Sarah bought 10 pears. (4/2 = 20/P solve and P=10)

500 dollars and its probably more than that