A marine biologist monitored the concentration of microplastics in a bay. She took 80 water samples from random locations and found the mean concentration to be 54 mg/L54 \text{ mg/L}54 mg/L with a standard deviation of 15 mg/L15 \text{ mg/L}15 mg/L. After a new filtration system was installed at a nearby treatment plant, she took 120 more samples and found the mean concentration to be 49 mg/L49 \text{ mg/L}49 mg/L with a standard deviation of 8 mg/L8 \text{ mg/L}8 mg/L.
Test at the 1%1\%1% level of significance whether the mean concentration of microplastics in the bay decreased after the filtration system was installed. State your hypotheses clearly.
Explain the significance of the Central Limit Theorem to the test in part (a).
State an assumption you have made about the sample variances to carry out this test.
616 exam-style questions on Edexcel A Level Maths The Normal Distribution, covering 3.1 The Normal Distribution, 3.2 Finding Probabilities for Normal Distributions, 3.3 The Inverse Normal Distribution Function, 3.4 The Standard Normal Distribution, 3.5 Finding the mean and standard deviation, 3.6 Approximating a Binomial Distribution, and 3.7 Hypothesis Testing with the Normal Distribution. Each one has a worked solution and a mark scheme showing where the marks go.