7) Using the table and information, choose the correct answer. A student notices that he does better on tests if he sleeps for at least 8 hours. He decides that for the next 3 tests he has in science class, he will sleep different amounts of times and see how he does on his tests. For his next science test, he sleeps only 4 hours the night before, 6 hours before the next test, and 8 hours before the final exam. The chart shows his data. Test Hours of Sleep Grade 1 4 56 2 6 75 3 8 80 What is the independent variable? grade hours of sleep science test

Lake happy times contains 100 km3 of pure water. it is fed by a river at a rate of 50 km3/yr. at time zero, there is a factory on one shore of lake happy times that begins introducing a pollutant to the lake at a rate of 2km3/yr. there is another river that is fed by lake happy times at a rate that keeps the volume of lake happy times constant. this means that the rate of flow from lake happy times into the outlet river is 52km3/yr. in turn, the flow from this outlet river goes into another lake, lake sad times, at an equal rate. finally, there is an outlet river from lake sad times flowing at a rate that keeps the volume of lake sad times at a constant 100 km3 . (a) find the amount of pollutant in lake sad times at the end of 3 months. (b) at the end of 3 months, observers close the factory due to environmental concerns and no further pollu- tant enters lake happy times. how long will it take for the pollutant in lake sad times (found in part (a)) to be cut in half?