This lecture covers the conversion between decimal and fractional writing, focusing on how a fraction corresponds to a decimal number and how a periodic decimal corresponds to a fraction. It also explains the concept of division with remainder using the Euclidean division. Through examples, the instructor demonstrates the process of converting decimal numbers to fractions and vice versa, emphasizing the uniqueness of natural numbers in this conversion. An additional trick is introduced to solve the issue of periodic numbers by multiplying them by a power of 10. The lecture concludes with various examples illustrating the conversion process and the application of the division with remainder.