Answer:
 6 weeks
Step-by-step explanation:
At $200/week instead of $150/week, Nina's saving rate is 200/150 = 4/3 times the original rate. The time to save a given amount is inversely proportional to the saving rate, so it will take 3/4 of the time when Nina saves at the higher rate.
 (3/4)(8 weeks) = 6 weeks
At $200 per week, it will take Nina 6 weeks to save enough for her vacation.