A researcher would like to compare the effectiveness of two different methods for teaching third-grade mathematics. One group of students is assigned to Dr. Tang's class using method I and another group is assigned to Dr. Washington's class using method II. At the end of the school year, each child is given a standardized mathematics achievement test and the two groups of scores are compared. What variable is confounded with the key variable of interest?

Titus works at a hotel. part of his job is to keep the complimentary pitcher of water at least half full and always with ice. when he starts his shift, the water level shows 4 gallons, or 128 cups of water. as the shift progresses, he records the level of the water every 10 minutes. after 2 hours, he uses a regression calculator to compute an equation for the decrease in water. his equation is w –0.414t + 129.549, where t is the number of minutes and w is the level of water. according to the equation, after about how many minutes would the water level be less than or equal to 64 cups?