Correct for what? Grammar, naturalness? Many years ago in school, I was told that writing a numeral, for example "4" instead of "four" was lazy for any number less than ten. 1. There were far fewer professional athletes in the past. - ok 2. There were far more professional athletes in the past. - ok 3. The kinder he is, the more his mother is proud of him. - ok 4. The friendlier she is, the more everyone loves her. - ok 5. The more you study, the better the results are. - ok 6. Mary is 60kg, Peter is 70kg. How many kg is Peter heavier than Mary? Mary is 10kg lighter than Peter. Peter is 10kg heavier than Mary. - ok (I'd use a full stop/period, not a comma). 7. My dress is $10 more expensive than your dress. Your dress is $10 cheaper than my dress. - ok 8.Peter scored 4 points fewer than Mary. - ok 9. Peter scored 4 fewer points than Mary. - ok 10. Mary scored 4 more points than Peter. - ok 11. Mary scored 4 points more than Peter. - ok 12. Mary is 20 years old. Peter is 15 years old. How much older is Mary than Peter? Mary is 5 years older than Peter. - ok 13. Mary worked for 12 hours a day 10 years ago. Now she works only 8 hours a day. She works 4 hours fewer than 10 years ago. - ok (I'd say worked 12 hours a day. No "for". "fewer"? or "less"? four fewer hours, four hours less.) 14. Peter works 4 hours more than 10 years ago. - ok. (I'd add "now". Peter now works 4 hours more than 10 years ago.) 15. Peter finished his exam 15 minutes earlier than allowed time. - ok

15: Peter finished his exam 15 minutes earlier than THE allowed time. 13:Mary worked for 12 hours a day 10 years ago. Now she works only 8 hours a day. She works 4 hours less than she did 10 years ago. All the rest sound good to me.