Wikijunior:What can you use math for?/Buying Something
Buying something 
When you buy something you pay money and receive change. It is important to know that you have received the correct change. Otherwise you might pay more than you have to for the item. It can also be useful to understand fractions so you can attribute the right value to parts of a unit (like a slice of cake or pizza) or enable you to deal with lengths, weights and capacities and conversions. It is even necessary for you to be able to transmit areas or volumes, in case you buy terrain or need a new fish-tank.
Jobs involving buying something 
Many jobs are involved with buying things. In fact, there is a type of job called a "purchaser". This person is paid to buy things for a company. They try to find the best prices on the things the company needs. There is also a type of job called a "cashier." This person runs a cash register and sells things to customers. In fact, there are many different jobs that involve buying and selling things.
Math used when buying something 
There are several types of math used when you buy something. First, you use counting and addition used in order to tell how much money you have. Percentages and addition are used to calculate the purchase price. Then subtraction is used in order to figure out how much change is due.
You go to a shop to buy a doll. The doll costs $10.00. You take the doll to the cashier. The cashier figures out the selling price by adding the cost, plus the tax. If the tax rate is 7%, the tax would be 70 cents on $10.00. So the cashier adds 70 cents to the 10.00 for a total sales price of $10.70. You take your money out of you pocket and count one ten-dollar bill and one one-dollar bills. Adding these two bills together gives you $11.00. You give this money to the cashier. She subtracts the selling price ($10.70) from the amount tendered ($11.00) and you receive 30 cents in change. The cashier gives you 30 cents and puts the doll in a bag and hands it to you.