Hi I wanted to know if I buy the 200 questions will it help me to improve my score in the quant. Are they the toughest questions on the Gmat.

I'm not sure what you mean by "200 questions." Can you clarify?

The only thing I can think of is the challenge math problems? If so, most of them are harder than the hardest questions on the GMAT. I don't recommend doing those unless you're already scoring 750+ (or the equivalent of the 99th percentile on the quant section alone). (And if you're already scoring that well... you don't exactly need the challenge problems...so, basically, they're not the best use of your time. :)