Think about what the expression actually says:
"There is twice as much A than B. Think about it:
The statement says that there is more A than B. Twice as much more. This means that more than 1 B will be needed to make 1 A.
If there is twice as much A as B, then you need 2 B's to have the same amount of As.
2B = A
"There is half as much C as A".
Here, the statement says that there is less C than A. One-half as much. This means that you'll need some amount less than A to have the same amount as C. In fact, if you cut A in half, you get C.
(1/2)A = C
Statements like these should be intuitive to you for success on the MCAT.