An EPA contractor needs to test the concentration of a substance in ten samples of the ground water. Seventy samples were already collected from a single source over the course of a week at regular intervals. The contractor wants to test samples

evenly over the course of that time frame.

Systematic sampling

Step-by-step explanation:

An EPA contractor needing to test the concentration of a substance in ten samples out seventy samples provided form a single source at regular time intervals.

For the contractor to effectively carry out the testing he has to employ a Non-probability system of sampling which is called Systematic sampling, because in systematic sampling we can assume the population size (x ) which in this case is 70 and the sample size( n ) to be 10

He will have to select ;  ( x / n )^th of the sampling frame for best results

if you are saying purchasing 3 social media ads and one newspaper ad counts as one purchase then there are:

6 purchases

if you are asking how many of each then:

18 social media ads

6 newspaper ads

the area of the rectangle is 35units^2.

step-by-step explanation:

graph it and find the distance.

it is a 5*7 rectangle.

therefore the area is 35units^2.