They are always even numbers. [Other variants possible.]

moderate

c)

i)
ii)

1 orange; 6 lemons
3 oranges; 3 lemons

moderate

Next steps:

Extension
This item ends itself to an extension, covering concepts of divisibility and pre-algebra, with children working in pairs, agreeing on the prices of oranges and lemons, and giving each other different sums of money to spend.