You have $21 to spend on prawns and potatoes. Prawns cost $20 per kilo and potatoes cost $2 per kilo.  

(a) Supposing you can buy as much or as little as you want of prawns and potatoes and you will spend the entire $21, then write down your budget constraint. You may simply write out the equation or draw the budget line. You don't need to do both.  

(b) If you buy 2.5 kilos of potatoes, how many kilos of prawns can you afford?  

(c) If your wants are to have twice as many (kilos) of prawns as (kilos) of potatoes, show that you would buy 0.5 kilo of potatoes and exactly 1 kilo of prawn.  How much money will you spend on each? Remember you must spend exactly $21.  

(d)  If the price of potatoes rises to $3 per kilo and the price of prawns falls to $18 per kilo, then can you still purchase the quantity of prawns and potatoes you chose in (c)?

Based on your answer, argue that your welfare is likely to be greater in this new situation where the price of potatoes is higher, but the price of prawns is lower.