Did you read instructions?
Okay I will explain in simple words...
Suppose I have a pizzeria...
I take orders of pizza by asking two questions..
What type of topping they want.
What type of Crust type they want
If they give me details ,I give them pizza !
Then in diary I note a message and also write order number.
suppose when I open the shop, no one gave order so I put order as 0
then someone comes and gives order, I increase it by 1.
Then if again someone comes I increase it by 1(now it becomes 2) and so on...
This is what first part of program is trying to accomplish.
Every pizza costs $7.5.
If someone give 3 pizza orders, I will take $7.5*3 ($22.5)
This is our second part of code is telling us to do..
But I also take tax on purchase (6% of tax )
This is what third part of code trying to accomplish.
at last I add total cost + tax on it and add it.
This is done by fourth part of code.
we have written a function to deal each part.
does it make sense?
This link may help you.