You need a systematic approach to solve this problem. Moreover do not assume anything from your side. What is stated in question is only true, things that are not stated should not be assumed. Also try to reword the problem so that you can understand it better.

Like if you are given a integer. Do not rule out -ve, +ve, as well as 0.

Moreover go to each statement one by one and see if you can put them up in a mathematical form or not. If you can then that will help you in combining both the statements.

E.g. take the wood log case.
Question says that 7 feet wood is cut into three pieces, say their lengths are x,y,z.
So as per question x+y+z=7

Statement 1:
tells you x+y=z(assuming that z is the longest piece)
substituting it in original equation we have 2z=7 => z=3.5 feet
we have length of longest piece but length of other 2 pieces although 3.5 can be any thing like 1.5, 2 or 1, 2.5. so length of all 3 pieces cannot be determined by this statement alone.

Statement 2:
Tells us that smallest piece is 0.5 feet. but does not tells us about length of other 2 pieces, so this is insufficient.

Combining 1 and 2:
From 1 we have longest piece = 3.5 feet, & x+y=3.5.
From 2 we know x=0.5(assuming x is the smallest piece)
=>y=3.5-0.5=3 feet
We have length of all the pieces.
So together both can answer the question.

Answer C.