y = 50 + 4x Here, 50 is the fixed number, or fixed 'cost' if we use money as an example. Here, 4x is the variable number, i.e. the number that changes depending on the value of x. For example: a) You decide to rent an apartment. If you want Wifi, the realtor tells you that it will cost $50 plus $10 for each wifi router. So, if you decide you want wifi, and need three wifi routers, it will cost a total of $50 + 4($10) = $90.00 b) You are making cupcakes to sell. You decide to open a business. You need to buy an oven. The oven costs $50. The cake batter costs $10/batch of cupcakes. Therefore, this equation (y = 50 + 4x) will help you decide how much it is going to cost to make your cupcakes. The fixed cost is $50 for the oven. The variable costs are 10x (i.e. $10 times the number of batches of cupcakes you want to make). If you decide you want to make 10 batches of cupcakes, total cost will be $50 + $10(10) = $150.00 c) You are planning a party. 50 of your friends are definitely coming. You tell your mom that she can invite some of her friends too, but only in groups of 4 because the extra tables you will be renting seat 4 people and you don't want to waste any money at all by having empty seats. You tell your mom to tell you how many groups (of 4 people) she will be inviting. When she says 5, you can easily determine how many people will be at the party. Total attendees = 50 + (4*5) = 70 people total.