Which of the following principles of experimental design reduces errors due to chance by repeating an experiment on different subjects under different conditions?

What is the value of x in the equation 2(x-4) = 4(2x + 1)?

How many glue sticks are in a basket containing 96 scissors, if the ratio of glue sticks to scissors is 19 to 57.

The random variable x represents the number of phone calls an author receives in a day, and it has a poisson distribution with a mean of 8.7 calls. what are the possible values of x