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 Royces (y) which take 2 people and 4 sheep.
He must not use more than 15 cars altogether.
a) What is the lowest total number of cars he can use?
b) If it costs $10 to hire each Fiat and $30 for each Rolls Royce , what is the cheapest solution?