If she spent 1/2 OF HER POCKET MONEY on the book and 1/6 OF HER POCKET MONEY on the movie, and you want to find out what part OF HER POCKET MONEY did she spend on the dress
Amount she spent on dress
= 3/4 - 1/2 - 1/6
= 9/12 - 6/12 - 2/12
= 9/12 - 8/12
= 1/12 OF HER POCKET MONEY
If she spent 1/2 OF WHAT SHE SPENT IN TOTAL on the book and 1/6 OF WHAT SHE SPENT IN TOTAL on the movie, and you want to find out what part OF HER POCKET MONEY did she spend on the dress.
Amount she spent on the dress
= 3/4 - (1/2 x 3/4) - (1/6 x 3/4)
= 3/4 - 3/8 - 1/8
= 6/8 - 3/8 - 1/8
= 2/8
= 1/4 OF HER POCKET MONEY
Stating what the fraction is part of is very important, or else the answer can be affected. So choose which one your question was asking