When we use mathematics to solve problems in the real world, we often talk about this as modelling. We start with a problem in the real world, and turn it into a mathematical problem that we can solve using mathematical tools. We then take the answer to the mathematical problem and translate it back to the real world.
Sometimes we can obtain an exact solution to the problem, but often the real-world problem is rather complex, with many factors to consider, and so we have to make some simplifying assumptions in order to obtain a mathematical problem that we can solve. An important part of the process is identifying which assumptions we can reasonably make, and then checking back to see whether the answer makes sense in the original context.