Question

A farmer has to transport 20 people and 32 sheep to a market. He can use either Fiats (x) which take 2 people and 1 sheep, or Rolls Royce's (y) which takes 2 people and 4 sheep. He must not use more than 15 cars altogether.
a) what is the lowest total number of cars he should use?
b) if it costs $10 to hire each Fiat and $30 to hire each Rolls Royce's. What is the cheapest solution?