Chris spent half of his money on a pair of headphones. then he spends half of his remaining money of recordable cds. finally, he spent his remaining $12.75 on a book. how much money did chris have to begin with?

Assume that Chris began with m dollars in his pocket.

Half of that, or m/2, went to purchase a pair of headphones.

Half of that, or (1/2)(m/2), or m/4, went to purchasing CDs.  By now Chris has spent (1/2 + 1/4) of his money:  (3/4)m.  He had (1/4)m left, and that (m/4) = $12.75.

Solve this for m:
         $12.75
m = = $5 (answer).    
            (1/4)

You should use a calculator for this.