In a cafeteria, a glass of lemonade, three sandwiches and seven biscuits have cost 1 shilling and 2 pence, and a glass of lemonade, four sandwiches and 10 biscuits they are worth 1 shilling and 5 pence. Find the price of:
a) a glass of lemonade, a sandwich and a cake;
b) two glasses of lemonade, three sandwiches and five biscuits.
(1 shilling = 12 pence). number theorysystem of equations