For example:

70 is 60+ 10.

80 is 4x20

99 is 4x20+ 10+9.

But if this is the difficulty for you to give up learning French, then you may have missed the treasure of French. After all, this difficulty can be fully mastered in a few hours, and it really shouldn't be a hurdle for me. The real difficulty of French numerals is not here, but that French numerals are in thousands, because French is grouped with three digits (like English) and Chinese is grouped with four digits. That is, 10,000 in Chinese is 10000 and 100,000 in French is 10000. The bigger the number, the more difficult it is to convert, and more deliberate practice is needed for French interpreters to be competent. Duan Zishou likes to joke about French numbers, but when he really learns, he can fully grasp the content in a few hours.