Jake spent 3/8 of his money on a dvd and 1/2 of what was left on a sandwich. he now has $6.25. how much money did he start with?

You can multiply 6.25 by 2 (which is 12.5) to find how much money he had before he bought the sandwich.  Then you can find how much money he had before buying the DvD by multiplying 12.5 by 8/3 which is 33.33 dollars.  Therefore Jake started with 33.33 dollars.
(you can prove this since 3/8 of 33.33 is 12.5 and 1/2 of that is 6.25)

I hope this helps.  Let me know in the comments if anything is unclear.