You might think it is very difficult to remember 15 to 20 digit numbers like 57484790954236.78 but there is a great two step technique which is effective and easy.
Step 1: you turn the numbers into pictures and you learn these by heart (it is worth the effort, since these will be your memory tools for the rest of your life) For instance:

As you can see, each image approximates a number 1=candle, 2=swan 3= , 4= a boat etc.. The technique is to make up stories with ridiculous imagery in order to remember the numbers.
Step 2: Once you have fully learned and fixed in your mind a chosen set of images for the numbers 0-10 you are in a position to remember long numbers.
What you do is split the numbers into groups of five (so 57484790954236.78 becomes 57484-79095-4236.7-8)
These then become little image groups.
57484 could be an eagle (5) conducting music(7) in front of a yacht(4) where a lady (8) is holding a model of a yacht (4).
Remembering that image group is a lot easier than remembering numbers. Fix the image in your mind and move on to the next set of five.
It is important to be consistent in which images you associate with which numbers.
Here is my list, but these might not work for you, so choose your own.
0 = copyright/writing
1 = candle
2 = swan
3 = mountains
4 = yacht
5 = man
6 = bomb/explosion
7 = conducting/music
8 = woman
9 = house/building
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