How many grams in a kg

What is the common difference in the following arithmetic sequence 7,3,-1,-5

An automated water dispenser fills packets with one liter of water on average, with a standard deviation of 5 milliliter. the manual says that after a year of operation the dispenser should be tested to see if it needs recalibration. a year later a number of filled packets are set aside and measured separately. it is found that the average packet now contains about 0.995 liters. does the dispenser need calibration? explain your answer.

What is the best way to answer questions with a x expression in the question. for example 3x + 10x=?