After eating a hearty meal they went to see the Buckingham palace. There, Rachel learned that 100 visitors came to the Buckingham palace on the previous day. If there were 666 visitors on that day how many more visitors visited the Buckingham palace on that day than on the previous day?
: 

There were 100 visitors on the previous day. 
There were 666 visitors on the day Rachel learned about the 100 visitors. 

That means there were 566 more visitors on the day Rachel learned about the 100 visitors than on the previous day.