Bunuel, obviously
I suggest you pay special attention to inequalities and absolute inequalities as these take a while to get your head around if you're not used to them. You may need extra practise as they seem to be popping up quite a bit recently in the 50/51 level DS questions and they're not that well covered in most books.
You need to get into good habits for DS from the very start. For
all DS questions involving unknown variables, remember the 4 horsemen of DS which are
- the variable(s) could be negative
- the variable(s) could be zero
- the variable(s) could be non-integer
- the variable(s) could be the same
You need to consider all 4 options
every time you do a DS question involving unknown variables. No exceptions. It must become second nature. You can NEVER assume that variables are positive, non zero, integer or different! Don't throw away questions because you forgot that a variable could be zero
You should also examine each DS question to determine whether you're being asked for a single discreet value (e.g. "what is the value of x?), a compound value ("what is the value of a+b?") or a yes/no question ("is x more than 10?")
For compound values, decide whether you need to know the exact values of a and b, the combined value OR how just they relate to each other (e.g if the question asks "is the product of a and b negative?", you don't necessarily need to know individually the status of a and b. If you can determine a and b always have opposive signs, or that a and b always have the same sign, then the actual individual signs of a and b don't matter)
For yes/no questions, remember that you often do not need to know the exact value of a variable. Don't fall into the C trap for these questions