Can you guide me through solving this problem: A farmer is selling ducks and chickens at the market. He sells ducks for $10 and chickens for $8. He sells 5 chickens and some ducks. He then spends half his earnings on a new wheelbarrow. After buying it, someone sees it and agrees to pay the farmer double what the farmer paid and the farmer earns another $60. How many ducks did the farmer sell?