Not because this topic is too difficult, but because it has a problem: according to scientific notation, simple notation in circulation primary schools cannot appear as you said.

The circular part of 0.78787 ... is 78, and writing a few more numbers is 0.78787878 ... The simple notation is: add a dot to 0.78 and 78;

The periodic node of 0.7878 ... is 78. Write a few more numbers to be 0.787878 ... The simple notation is 0.78, and add a dot to 78.

The two numbers you are talking about are actually the difference between the correct notation and the wrong notation of the same number.