3. this dress is less expensive than that one
=>this dress is cheaper than that one
4. this flat isn't as big as our old one
=>our old flat is bigger than this flat
5. mary isn't as pretty as her sister
=>mary's sister is more pretty than mary
6. nobody is happier tham miss hoa
=> miss hoai is the happiest
7. i have less homework than you do
=>you have more homework than I do