¿Qué significa There was a substantial increase in the percentage of families with only one child,while that of families with more children saw a slight decrease. Here,what's the meaning of "that of"？ ?

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“That of” refers back to “percentage.”

“There was a substantial increase in the percentage of families with only one child, while the percentage of families with more children saw a slight decrease.”



“That of” is used to compared two things without using the same word twice.

Examples:
The population of the U.S. is 328 million while the population of Canada is 37 million.
➡️ The population of the U.S. is 328 million while that of Canada is 37 million.


The politician tried to boost his own reputation and destroy his opponent’s reputation.
➡️ The politician tried to boost his own reputation and destroy that of his opponent.