I recommend you to study with 2 step target:
First target: Being able to solve those problems
Sample tests at McK website are great resources First, you can try to solve those questions without time constraint. Then check the ones you could not solve if there are any and make sure you understand solutions. At the point that you feel like you can solve those questions without time limit, you can pass to the next step.
Second target: Increasing your speed
There is some other sample PST tests available on different websites. You can solve those with a time limit and try to improve your speed.
Hope it helps