Functions are one of the most important concepts in mathematics. They can be used to describe (model) various situations and processes. To better understand the concept of functions, imagine a blender that makes smoothies. Here, the blender acts as a function, which in a certain way - grinding, achieves another result - a smoothie. A mathematical function takes one or more values (ingredients) and in a certain mathematical way (subtracting, adding, multiplying, etc.) transforms them into a new value (result). A linear function is the simplest type of functions, and it is a direct proportion between two variables. The linear function can be described with a simple formula y = ax + b, where "y" is the dependent variable, "a" is a constant (coefficient of the function's slope), "x" is the independent variable, and "b" is a constant indicating the function's deviation from the coordinate system's origin. We'll return to this. For example, consider mobile network providers' tariffs, where each tariff can be described as a linear function. Here, one variable could be the volume of data used, and the other - costs. Simply put, the more data is used, the higher the costs, and this relationship is directly proportional. Let's consider two different mobile service tariffs. Tariff "a" is five euros fixed monthly subscription charge and five euro cents per used megabyte. Tariff "b" is ten euros monthly subscription charge and three euro cents per used megabyte. Here, the variable component is the cost per used megabyte, and the fixed one - the monthly charge. Using the linear function and depicting its graphs, one can evaluate which tariff is more advantageous depending on the data usage volume. So, our task is to evaluate at what data usage volume Tariff b becomes more advantageous than Tariff a. Assuming x is the data usage volume (megabytes). Tariff a's costs can be calculated as 0.05 times x plus five, whereas Tariff b's costs as 0.03 times x plus ten. The task is to find at what x value Tariff b becomes more advantageous. Put the data into linear function and depict it in graphs. We see that the graph of linear function is a straight line (who would have thought) and both graphs intersect at the point with the coordinates (two hundred and fifty and seventeen point five). Here it is clearly seen that the tariff b is advantageous in case your megabyte consumption exceeds two hundred and fifty, but if your consumption is lower – the tariff a is more advantageous. The linear function is the basis for many more complex mathematical concepts. Its understanding is crucial not only in mathematics but also in many everyday situations as well as in other science branches. Likewise, mastering linear functions, you will significantly improve your algebra knowledge, which is important both for academic and professional development.